How To Learn To GM
There are a variety of resources nowadays that can help you accomplish this. There are also many existing questions on this site about GMing that will point you to more content than you can ever consume.
Watch
In your question, you mention wanting to see more examples of real play. There's a number of ways to do so.
Actual Play Resources
- Podcasts capture the entire play session. There's video podcasts too like on Twitch. See Where can I find actual play podcasts for RPGs?
- Session Summaries (aka Actual Plays, Story Hours, Campaign Journals) usually are severely abridged, but leave out a lot of the cruft. See Where can I find transcripts of actual game sessions? and Where to find game session reports?
- Blogs. There's a million blogs about how to GM. Start with the RPG Bloggers Network. Go to the blogrolls of blogs you like to find more like them. Focus in on blogs about your chosen game(s) and play style(s).
- Play by post forums. If you want to watch people actually play in text, there's a million of these too. Many dedicated sites, specific forums on RPG.net, ENWorld, Paizo, etc. In fact, RP-by-post is very popular even when not affiliated with a proper RPG/ruleset.
- Sit in. There are plenty of other people running games, some in public places like your friendly local game store (D&D Encounters, Pathfinder Society) and conventions. See below under "Play" though, if you're going to the effort of being there you need to stop being a wallflower and get on in and play.
Some games also have better advice sections than others - see What role-playing games have good gamemaster advice sections?
Play
In the end though just watching is not the most effective approach to learning. Watching games is less useful experience than actually being in one. Have you considered playing in those games before running them to learn from other GMs? It's reasonably easy to find other gaming groups, you don't have to abandon yours to play in another. Where can I find other RPG players?
Go to RPG conventions, find games at gaming stores, play on forums or G+ (see also Finding online RPG players for a play-by-chat RPG Campaign?) - just get more experience. The GM was often called the "judge" in the old days, and in the legal world you need to spend a lot of time being a lawyer before you make a good judge. You need to spend some time playing to become a good GM. If you can't think how the players will proceed in a given situation, you need more play time.
Read
There are many books on GMing - see What is the single most influential book every GM should read?
Also try watching/reading relevant genre media. "I don't get how to put together a story" should get its first-order correction by consuming some of that genre and looking at the stories.
Learn
A lot of the problem you seem to be experiencing is pure storytelling. Try How do I get better at narrating/storytelling as a GM? and As a GM, how can I create and role-play diverse NPCs better? Read up on the specific aspects of GMing you feel you're not good at, there's plenty here. Try questions tagged with the gm-techniques tag. Feel free and ask questions here as well about specific aspects of GMing.
There are also a large, large number of RPG forums out there in the world, for every game and type of gaming. If you don't understand something someone posts, you can easily reply and ask.
Do
aka How I Learned To GM
We didn't have these newfangled Interwebs when I was a kid. I GMed almost before I ever played. I did play in a very informal game of D&D in a car on the way to Scout camp, no dice, PvP, everyone had artifact weapons. But other than that, I started out as a GM. I bought a sci-fi RPG (Star Frontiers) without knowing anything about it (I had bought and played a little TSR chit game, Star Force, and was looking for other fun stuff from the same company). None of my friends were interested in GMing and I was in a small Texas town that didn't have conventions or whatnot - life was less mobile and connected back then. So I just read the game books and then ran games for my friends. And I kept running them, and learned from my mistakes and corrected. I read comics and science fiction avidly, so characters and plots weren't that hard to devise. Beyond that, I just learned the way you learn to do anything through practice, whether it's a sport, writing, a musical instrument... How-to's and YouTube videos are cute jumpstarters nowadays, but "Do, and learn from doing" has yet to be eclipsed in being the primary way to actually become good at something.
Fear of "making a mistake" is the dumbest and most paralyzing instinct you can have in life. In a video game you're going to die a couple times off the bat; in baseball you're going to swing and miss a lot before you hit; in baking you're gonna burn some cookies. But you learn through those mistakes. It's fine to do a little reading up ahead of time but the only way to become good, really, is get your butt in gear and do it.
I am a little bit nervous about trying to DM {...} Am I too new to try this? Should I wait a few more months?
This is perfectly fine. Everyone has to start somewhere. Try not to be nervous. Assuming your players are understanding, they likely won't expect much out of your campaign (which is a good thing when your are a new DM).
As a bit of information about "what it takes" to run a game, when I first started DMing with AD&D 2nd Edition circa 1992, I literally had never played any type of Pen and Paper RPG... and neither had the people I started playing with. So you're never "too new". :-)
With that in mind, here are some tips to get you started...
Read The Books
I think I would like to run a game as close to RAW as I can get.
This is good. Any system you run, you will want to know as much of the rules as possible even if you don't use them.
For D&D your reading list should be:
- the basic Player's Handbook
- the basic Dungeon Master's Guide
Read these cover to cover at least once before DMing a game. Likewise, you should really grab a copy of a Monster Manual and peruse that as well.
I do not own any core books, I mostly just get my info online.
Since 3.5 is part of the D20 system, you should have no issues being able to look through most of this online. To get you started, there is the SRD20, which includes updates for 3.0 and 3.5.
Note that there is also the Pathfinder SRD by Paizo Publishing. Don't be confused - Pathfinder isn't official 3.5. And while close in many ways, it is a different game system.
To be clear, the rules you should study are the SRD20 rules above. Unless you are running a Pathfinder game, the Pathfinder SRD is not what you should be reading -- but it may be suitable for a change of pace later on.
Besides dice & game mechanics, the parts you should really focus on studying prior to running your first game are simple combat & intiative, actions in combat, injury and death, general magic, ability checks and saving throws. These are the rules that are used most often and are the most important rules for you as a new DM. Almost all of the other stuff can be fudged.
As a final comment on reading, having as good a knowledge of these books as possible will aid you immeasurably... but don't sweat memorizing them. Your knowledge doesn't need to be encyclopedic and it's ok not to know some (or most) of the rules. But when things "come up" in your game (and they will), you will be much better prepared if you are well read.
Is the [Dungeon Master's] Guide mandatory for running games?
Erm, maybe?
The DMG isn't "mandatory" for running a game in the sense you can make up rules, but you will be sorely lacking if you haven't read it at least once and don't have a reference copy (even if it's a few select print outs or a laptop open to the SRD20).
Don't forget that often understanding DMs will likely let you borrow their DMG or other books as long as it doesn't interfere too badly with their own games/adventure creation.
What is in [the Dungeon Master's Guide]?
In D&D, the DMG is generally the "other half" of the rules assuming the Player's Handbook can be considered the first half (or the third part of a trifecta that makes up the core rule books if your count the Monster Manual).
Essentially it covers all the stuff the Player's Handbook doesn't -- what the players don't need to know to play but what you as DM (roughly) need to know rule-wise to run a game world.
That being said, the SRD20 link above covers everything you need to know as DM to run a game, and for these purposes, the DMG can be considered mostly "flavor".
How To Be A "Good" DM - Cliffnotes Version
While there are many nuances to being a good DM, arguably the most important things when learning how to DM are:
How to tell an interesting story with the help of your players (collaborative storytelling)
How to keep your players alive and NOT to kill them
How to keep your players happy (give, give, give!) without throwing your game out of whack
How help your players have fun (to the point they are saying "Wasn't it cool when...?")
To this end, there are a number of guidelines that generally apply:
Listen
Listen to your players and don't simply dismiss them. Remember, it isn't just your game -- it's yours and the players. You need to strike a balance that makes you and your players happy. If your players are subtly or overtly complaining on a regular basis, you are likely failing this test.
Ask some form of "What do you do?" as much as reasonably possible. Getting player's to help tell the story is an integral part of creating a fun game.
Say yes to and incorporate player input about the game. Whether they want to try a cool non-standard action, have thoughts on their own characters or the game world or are giving hints about how they think the plot might unfold or what they may want for their character, say yes -- unless (and only if) it would break the game too badly.
Player input often makes a DMs job of creating a world (and a fun game) much simpler. Don't waste it. Talk to your players regularly before and after sessions. Don't ignore unresolved play issues.
Decide How Your Game Operates For Maximum Fun
Don't be a rules lawyer and don't let your players be ones either.
Don't be slavish to the Rules-As-Written if they are no fun. Don't let player's force you into doing things solely for their advantage because of how they interpret rules. Do listen regarding player interpretations of the rules and consider carefully what they are saying (especially if they are pointing out something valid and correct... or if it may add fun to the game).
Be firm and consistent about how your game works. Fluid games are fine, but games with fairly firm guidelines on how things are resolved in-game (by the book or house rules) are easier for a DM to manage (especially a novice one). And remember that you should know the rules (wherever they come from) before you break them.
Use a DMs screen to hide dice rolls and other DM-only information. You lose a lot of flexibility as a DM to control your game world by rolling in the open. If players don't trust you to role behind a screen, there are bigger issues you need to deal with.
Likewise, don't show and tell monster stats. Here is the reason why I suggest this approach.
Get Encounters "Right"
Easier is better. If your players are coming close to (or are actually) dying even once a game, you are likely making things too difficult for them. Players will complain about things being too easy, but will hold grudges and leave a game that's too hard. When in doubt, dial it down.
Start off slow with treasure (gold & valuables), magical items, and experience. There is an art to giving these out and one of the easiest ways to create havoc in your game is to give out too much too soon.
Try not to leave any player on the sidelines for too long. Try to make sure everyone benefits from an encounter (both with fun and with useful items).
Your job is not to punish players. Don't take things they do or say personally. You don't need to "train" your players or get revenge on them. If you aren't having a good time DMing, you can always stop running adventures or find another group.
Use What You Know
Think of all the times your were bored, upset, confused, felt left out or otherwise weren't having fun. Try avoid these situations when running your game.
Think of all the times you have really had fun playing as a character. Then as a DM, try to create similar scenarios in your own adventures. Your players will likely have as much fun as you did.
Watch players. They will tell you minute by minute about how much fun they are having in-game by things like tone of voice and body language. If things are dragging, trying to figure out a way to pick things up and move the story along (create interest).
Advanced DM Tips
Learn to properly use the Magician's Force and dice fudging.
Despite the very first two items (listening and asking), and the fact that RPGs are collaborative things, there will be times when you need to have things happen in your game.
In addition, at times, you will need to help players without appearing to do so (such as when they make terrible mistakes that, if you didn't manipulate the outcome, would be no fun for anybody). Don't do it often, and certainly as a novice start small, but try to practice these skills when you can.
You will fail. No DM has ever run a game where everything went perfectly. Your job as DM is to keep the players having fun, but you won't always succeed. Just keep at it, and, as a novice, don't be too hard on yourself -- let your players decide how much fun they are having.
Running Your First Adventure
Since your a novice, even though you may be playing with people who are more experienced than you, it's probably a good idea to keep it simple. Don't get into any grand adventures just yet.
Like others have already stated, start small. 1st - 3rd level characters are likely best. At these levels, there are far fewer powers and abilities to deal with, and monsters have fewer abilities as well.
As far as an adventure is concerned, start with a simple one you can play over 1-5 sessions without getting too much into the idea of a grand campaign. The simpler, the better. You can always build into to a larger story arc later.
Regarding the subject of creating an adventure or DMing a pre-made one, as a new DM I would choose the latter (pre-made). Anything that's even of moderate quality will give you a much better idea and potential blueprint of how to design your own adventures well.
It will also give you a very early handle on what do when your players inevitably stray from the things you have planned for them and how to come up with things you have no plan for at all.
If you insist on making your own adventure(s), you will likely need to learn about (or at least consider) the following things:
Basic plotting (e.g. 1. Party gets into town 2. Party hears about local bandits 3. Party defeat bandits besieging town but 4. Find a clue that it was the mayor who hired the bandits in the first place... 5. ??? 6. Profit.)
Planning major and minor NPCs (stats, general personality, what they do in a limited number of scenarios -- i.e does the mayor spill the beans if confronted, what happens if the party simply attacks him?, etc.)
Location design & mapping (elementary medieval services as well as fantasy tropes such as magic shops plus multiple special locations -- mayor's office with secret door, bandit cave, etc.).
Real estate planning. Believable (and fun) settings include a variety of man-made and natural locations. There is nothing wrong with themed quests, but exploring a bog-standard dungeon every single game (even an upgraded one with shiny new monsters and even more devious traps) will likely get boring for everyone rather quickly.
Designing in-game materials (e.g. bandit treasure map handout for the players found in the bandit cave.)
Handing out treasure and experience (per encounter and per adventure)
Creating world flavor (e.g. do the the bandits have a theme song? ...dunna-dunna-dunna-dunna, dunna-dunna-dunna-dunna, baaandits!)
Creating notes for yourself (and then learning about how to properly ignore them)
Managing your time (how many hours a week do you really to want to devote to this?)
Roleplaying everyone except your cleric
Thankfully, these are all much easier at lower levels and on smaller scales.
Four things that are absolutely indispensable when creating adventures are monsters (creatures and NPCs), monsters with treasure, treasure tables and magic items. All your other DM tools (such as town maps, NPC resources, etc.) will start small and likely grow larger later as you design and run more adventures.
Building and Running Encounters
Building and running encounters shouldn't be too difficult if you keep the following in mind:
Make combat encounters easier, not tougher. In game, if a combat encounter proves too trivial, it's much more believable (and simpler) to have reinforcements or the like arrive than to find a cheesy way to kludge your way out of an obviously overwhelming battle.
Combat encounters should be snappy in most cases. The only time you should have a two-hour combat scenario is when your players are determining the fate of the world.
Good encounters have tactics. Ideally, creatures and players should be able to use the surrounding environment to interesting effect.
Keep a close eye on treasure. There is almost no quicker way to create game imbalance than to give players too much money (gold and valuables) or too much magic.
Traps and obstacles (man-made or natural), riddles and puzzles are also encounters. Not every interaction the party has has to be between themselves and NPCs/monsters. Let the party exercise their noggins a bit and interact among themselves.
Dilemmas are encounters. Creating situations where the party needs to talk to each other due to different methods of approaching a problem is often a great way to mix things up.
Non-combat encounters are encounters too. Whether it is talking to an NPC, or witnessing and investigating a strange event, not every encounter has to involve mortal peril to be fun.
Encounters are not filler or time wasters. Make encounters count. Ideally, they are set pieces that A) Give the party something(s) they need (experience, treasure or information) B) Move the story forward in an interesting manner or C) Allow the player to have an experience for the sake of fun.
Lastly, Get Help From The Pros
I am planning on asking my current DM to play, so he can give guidance and support if necessary.
This is an excellent idea.
Make sure to listen to their opinion if it sounds reasonable to you, or you are otherwise really in the dark.
Don't be afraid to pull them aside and explain what you are trying to do, especially if things go too wrong in-game.
Don't be too afraid of spoiling at least some of the plot for them if they need to know.
All in all, learning shortcuts from a good DM at the outset will save you lots of trouble down the road.
Best Answer
Congrats on deciding to overcome the challenges associated with your condition!
Your condition, while rare, is not an automatic no-GM situation. Here is some advice for successfully world-building for your RP scenarios and campaign despite lacking the "visual mind's eye" (please note, a lot of these will take effort):
Collect (or Create) visual resources
1) Search engines are your friend
Use numerous image search methods with varying keywords related to what you are trying to go display. Need to describe a castle? Find images or videos of castles. The same can be done with characters, items, costumes, jewelry, and so forth.
2) Fiverr (or similar) is also your friend, if you are willing to spend a bit
If you have a bit of cash, then you can ask artists to create what you are looking for, or modify something you found that is 'almost'. When asking others to modify a pre-existing thing, however, be aware of copyright, license, and usage restrictions.
3) Photoshop or GIMP is your also your friend
If you don't, or can't, pay someone else to do it, give your own artist's chops a try. Even when lacking a visual mind's eye, with the undo features of modern graphics software, you can alter (and mess up) to your heart's content, and still undo anything you don't like how it turned out. Just remember to save often. As a tip, save alterations under different names, so that you can go back to a specific point in time - takes more file space, but allows you more latitude in your work. GIMP, by the way, is free.
4) Bryce, Daz3D, Blender are also your friends
Daz and Blender and are free 3D software which are quite capable in their own respects. Strong communities with lots of tips, tricks, and advice support both; Blender even has a Stack Exchange site. Bryce is not free, but is usually worth the price tag - and is still considered one of the leading landscape creation tools. In conjunction with one of the other two (it already is configured to work directly with Daz) it can support buildings and characters as well. Use existing resources (lots of free stuff out there online) and build your scenes (towns, buildings, maps, you name it), or even learn to make your own resources.
5) Use Props
Always fun to have the real thing (or close approximation) handy for reference. Nothing says light saber like some red and blue giant pool noodles, duct tape, and electrical tape. Plus you get to bash each other after (or during) game for fun (and profit?).
On a more serious note, a yellowed piece of paper with the note the characters just found, a Chinese puzzle box with something rattling inside of it for the players to actually solve, a cracked chess piece with what looks like a blood stain (a bit of paint or even certain types of markers work), can do wonders to set a scene and tone to the game.
These visual collections can be used to 'show' what you want, can be composited to 'create' what you want to see and/or show, and is all around useful for both you and your players. When you find a picture that you like, decide it is a picture of the Plain of (random name) and just behind that out-thrust rock is the City of (another random name). You can show it to the players if appropriate, or simply use it for your own personal reference to aid your descriptive verbiage.
Collect (or Create) aural resources
For those with a visual mind's eye, a bit of background sound will help them build a scene. If your locations and/or characters have their own theme music, or associated background sounds, a bit (or a lot) of work with some music software or websites where musicians post their work freely for everyone to listen to (or spend some resources to collect music files and/or sound clips) and you can have your own SFX library at your command.
Collect Descriptions
If you find that your aphantasia extends to preventing you from coming up with verbal descriptions of visual effects, then there are still a couple of things you can do. You can read the works of others, and collect quotes. Find bits of verse and descriptive text, make a copy and keep a portfolio of useful descriptive bits. When planning out adventures and locations, base descriptions on certain of your snippets. To keep your descriptions consistent (and unique to each location), keep notes so that you don't overuse any particular snippets.
Also, walk around (or use Google Maps Street View for distant locations) and verbally describe to yourself what you are seeing. Write your descriptions down. Touch things (appropriately, mind you), smell things (again...), take pictures (yet again...) and listen; in other words gather sensory data, and practice putting it into words. Describing the smell of a place as rank, musty, but with just a touch of the lingering memory of fresh bread, and your non-aphantasia players will build the image without any further help from you.... Plus, it's a great excuse to get out and see the sights, visit museums, and other places of local color.
If you can record the sounds and images of your walkabouts, this can help with your later descriptions as you (probably?) can't pull up the visual memories of your experiences.
I hope this advice is helpful.