This will unfortunately require a bit more work than you probably want to do when running the starter, but it's doable. You need a copy of this document. Take a gander at the second to last chapter and there you will find the encounter construction rules.
Basically what you need to do is find the encounter construction rules, reset the XP budgets to be for 6 players, and then rebuild each encounter with the appropriate number of monsters.
I would note that this is completely optional. You can run this straight up, let your party mow through it, and simply chalk it up as a learning experience.
One last suggestion if you don't want to change anything at all, is to adjust monster HP and damage. This is something I've experimented a bit with in my games with fewer people, but it's something that is definitely worth trying as monster HP and Damage are significant scale factors in 5e. Each monster has their average HP and damage listed. use a midpoint between their average and their high.
You
I'll deal with your issues first: you are an angry 14 year old.
Don't sweat it; everybody was, is or will be. Maturity can in fact be summed up as learning not to punch the face of someone who richly deserves it.
You have to remember that you have no control over the way other people behave; you only have control over the way you behave. And ... you behaved badly. People say "I lost my temper"; this is a euphemism that means they choose to express anger in an aggressive and non-productive way. Learn to choose to react differently; no one ever changed their mind by being yelled at - at best you can get someone to back down and resent you.
Oh, and because none of us is perfect and I have chosen to "express anger in an aggressive and non-productive way" myself (and no doubt will in the future), you need to know how to recover from that. This is easy: it's called an apology. There are 2 reasons we say sorry and both are applicable here: 1. to show that you know that you behaved badly and 2. to repair the hurt you caused other people.
Him
This guy was rude; there is no doubt about that. You haven't mentioned his age or the age of the group but if this guy was an adult then this is really bad behaviour; if there were adults there who didn't intervene then this reflects poorly on them too.
All the stuff I said about you is equally applicable to him; of course, he's not reading it.
Due to the wonderful diversity of humanity you will, from time to time, encounter people who are rude. They may be rude because they are: tired, drunk, just had their dog die, just got fired, have a splitting headache or are just obnoxious p*%^ks who didn't have enough parental discipline growing up. Notwithstanding, dealing with rude people is a skill and, like any other skill, you can learn it.
Here's a quick quiz. In response to his opening remark of "Let's get this over with, I got stuff to do.", which of the following is likely to give the best outcome:
- Ignoring it
- Beating him to death with your dice bag
- "Sorry we're keeping you, why don't you leave now?"
- "I was planning on a 3-4 hour session. What time is your other appointment and we'll see what we can do to accommodate it."
No 2 could be fun but No 4 does a lot with a great deal of economy, it:
- doesn't let the rude remark slide through unremarked
- shows that you care about his problems (both the other appointment and his rudeness)
- establishes expectations on timeframe
- establishes your authority
- enables the group as part of the solution
- shows what a nice guy you are.
Note: even if you say this you don't have to accommodate him! Polite and nice are not the same thing.
Expectations
When you sit down to play chess you know what you are going to get but when you ask someone to play football and you come ready for soccer and they come ready for gridiron; you have a problem. Ways of playing D&D range from treating it as a tactical war game to be won to using it as improv theatre and everything in between. All of these are valid and it's you job to give the players (including yourself) what they want out of the game.
Take 5 minutes next time you meet to find out what type of play each player prefers and describe your preferred style to them.
I think of agency as:
Players making informed decisions that have reasonable consequences
The D&D 5e Player's Handbook neatly encapsulates this in the "How to Play" section on page 6 and it is applicable to all RPGs that have DM/GM (some don't):
The DM describes the environment
The players describe what they want to do
The DM narrates the results of the adventurers’ actions
"I kinda got nerdy and got really into setting the scene. It was an opening narrative, so it took about 4–5 minutes for me to give all the backstory to the local area."
Cool, at what point did this lead to step 2: The players describe what they want to do? How much of it was describing the choices they could make? How much of the information was relevant to those choices? RPGs are a dialogue, not a monologue; unless the player's feel like they are involved in the storytelling then they might as well be at a book reading.
Back story is fine but if it was good it would be part of the story; the reason it is back story is that it is not enough fun to be the story. Back story may need to be there (or not) but it should emerge from the play; not be read at the players.
Scene setting is about providing just enough information that the player's can see the choices and have enough information to intelligently choose one; remembering that choosing to seek more information is always one of the choices.
For example:
It's a dark and stormy night when the captain puts you ashore in front of the imposing wall of the jungle. "This is where I left your friend; I'll be back at dawn five days hence. Don't be late!"
The beach stretches off in both directions, a narrow strip of sand bathed in moonlight. There are no obvious paths into the undergrowth. Sailors are carrying your possessions off the longboat and dropping them above the high water mark.
What do you do?
The scene is set and its now the player's turn.
Do they have enough information to make intelligent choices? No but you have handed them the initiative and they are now free to ask questions about what interests them. They might:
- Talk more to the captain
- Ask about their "friend"
- Ask what supplies they have
- Ask what the jungle/beach looks like
- Decide to troop straight off into the undergrowth (your job involves enabling idiots, too)
- Cast some long duration spells
- Cast Fly to take a look round
- Do something neither you nor I have thought of which is where the ultimate fun of DMing a RPG lies - dropping players into a situation and seeing what they do to it.
Now it is possible that some player's will not be comfortable with taking the initiative but the overwhelming likelihood is that within the group one or two will shoulder leadership roles. If so, they can start the ball rolling; I would suggest that you ask the other players "Are you happy with that?" to up their level of involvement.
In the unlikely event that no one seizes the opportunity, you can go on to enumerate what you think they can do. Some people like to pick from the menu rather than having to write it themselves first.
Best Answer
First of all, congratulations on noticing that some people are having a problem and trying to find a solution to it. Most of the dungeon masters I meet just ignore the quiet people who are not enjoying the game and focus on the party's face.
I had similar players before and I spoke with them in private and asked them what I could do to make the game more enjoyable for them. One of them said they just 'weren't feeling it' while another one said that I was not paying enough attention to people who were shy.
In any case, my advice is to speak with them or try to make them feel included by tailoring encounters where their character will shine should you prefer to not to directly confront them.