You can play any way you want to and do not have to jungle. However, some champs are so good at jungling, that one usually prefers that to normal laning (Amumu, Shaco, Skarner and the like).
You are right, that a jungler means that one other player is left alone against two enemies. However, he has the protection of a turret, which is enough in the early game to survive. Plus, he levels almost twice as fast as his two enemies, so he will very soon have his ultimate available against two level 3 or 4 enemies. Of course, some champions have a hard time to stay alone in a side-lane. Make sure when you have a jungler in your team, that you have a solo-champ that can hold its own without problems.
Jungling can only theoretically be done by all champions (sure, anyone can go into the jungle and attack creeps there). However, most champions are way too slow to make it worthwhile. They would profit much more from staying in a lane. These champions should not try jungling. If you want to try jungling yourself, then select a champion, for which you have one of these jungling guides at hand. Don't just try any random champion out in the jungle.
Finally, the most important point about jungling is one that most novice players often forget. You do not jungle in order to remain in peace and level up on your own. You jungle, so you can level up and at the right moment appear in a lane for a surprise gank. Enemies have to be constantly aware that a jungler may pop out of a nearby brush all of a sudden turning their 1v1 or 2v2 lane into a 2v1 or 3v2 problem. If done right, jungling can very quickly devastate your enemies, because the jungler has the power to turn around any lane. If one of your team's lanes is having trouble, adding another champion to it will devastate the enemies and at least break their attacks. Often, you can even net a kill or two. If you're a good jungler and help out frequently with ganks, then you will soon outlevel your enemies and the game can be pretty much decided in less than 20min.
So in summary, do not doubt jungling. It is a very popular and strong strategy. However, be aware that being a good jungler is not easy. Especially, when playing together with other novices, who may not know how to deal with a jungler in their own team. I suggest, you try to get at least several dozen games of normal lane-gameplay done, before you try out jungling yourself. And then, start with practice games, so you are certain, that you will survive the early jungle farming. It is most embarassing to die to the golem when no enemy was ever present.
The better option in this case is to avoid falling behind so far in the first place — being level 8 when others are level 15 is a huge gap! To that end, I'd suggest buying more defensive items early game (such as items that give HP, Armor, or MR). For example, as an AP champion, go for an early Rylai's Crystal Scepter instead of dying multiple times until you save up for Rabadon's Deathcap. For AD, I like to go for early Zeke's Herald on heroes that I'm bad with, because it:
- adds HP, life steal & AS
- allows you to support allies with an aura even if you're too weak to help directly
- adds CDR, which is important if your main contribution is to blow your skills to snare or damage and then run away so you don't die
The above points still don't answer what to do if you are already way behind. Honestly, at this point, there is likely a skill difference that will prevent you from doing much anyway, but let's focus on what you can do.
Item choices
Don't blindly follow a guide build order if you're having trouble.
+HP: Confused about whether Armor or MR would be better? Too poor to afford both against a mixed-damage team? Adding HP is a general solution that boosts your survivability for those nasty burst attackers so that you have a chance to get away. Note that %HP damage is your natural enemy in this case - beware of Madred's, DFGs, etc.
Aura Items: You're a wimp, but your allies aren't (or else the game would be already ending). Help out with Zeke's for sustain, Frozen Heart to counter enemy ADCs, etc. — all while remaining a safe distance for quick getaways.
GP10: These items are best purchased early game. I don't suggest buying them when you are thinking of playing catch up (especially after laning phase is finished) unless you expect the game to take a long time.
CDR: On any hero with good support spells (e.g. Lux), Cooldown Reduction is vital for more chances to help out. For instance, Lux can kill runners or deny areas with her R more often, and Lulu can save allies from ignite or DoT spells with her E or R more.
Game Play
- Never engage 1 vs. 1 when you're underleveled.
- Try for assists - can you snare an enemy for your ally to finish off? Even if you just Exhaust him and run away, you can pick up a valuable assist for some free gold and experience.
- Don't ignore neutral creeps. Level 8 heroes can easily kill wraiths & wolves for easy gold/xp if the creep wave is pushed out or guarded by the enemy champion.
- Aggressively clear enemy creep waves when there are no champions around to stop you. This helps push your minions to assist in killing their towers, and you could distract an enemy to come over to chase you off (in which case, you should run, because you're weaker... but putting a stronger hero out of position is also good!).
- Spam AoE and even ultimates to clear creep waves if there is little likelihood of a teamfight in the next minute (and you being within range to help).
- Unlike enemy champions, neutral buff monsters (blue/red) don't switch target to weaker heroes. Help by casting shields on allies taking blue buff, dragon etc.; this saves their HP for fights while you can go back for a mana refill.
Best Answer
A sightstone costs 950g. It gives you 180 health, the same as the ruby crystal that goes into creating it. Assuming the ruby crystal is of value to you, that means you have 950g-475g=475g invested in the warding portion of the sightstone alone. If that health isn't of value to you, or you would not have gotten a ruby crystal, then you should really not discount (all of) the ruby crystal's price.
This means you have somewhere between 475g (best case) and 950g (worst case, because you don't value HP at all) invested in the sight stone.
Green wards cost 75g. 475/75 = 6.33 wards. 950/75 = 12.67 wards.
In general, if you use it 7 or more times (meaning going back once after you buy), you've probably made back your cost. If you don't value health much, think of it more like 10 uses to break even (go back twice). If you don't value health at all, 13 uses (go back three times).
As a further consideration, remember that you can sell it back later in the game, if this turns out to be useful for you. This could reduce the number of wards you consider your "break even" point, especially if you don't value the health from the item. If you had bought an equivalent number of wards, you would not be able to recoup the sell price of the sightstone later in the game (380g)
EDIT: With regards to upgrading, the sightstone only upgrades into the ruby sightstone. This adds the effect of an additional ruby crystal for MORE than the price of a ruby crystal (price of crystal + cost to combine). While this is a useful thing if you're out of item slots and have gold to upgrade, it doesn't really apply to situations where you'd be worried about efficiency. It does allow you to have more charges (5 instead of 4) so you wouldn't have to back as often, and as pointed out in the comments allows 3 simultaneous ward placements.
Along the lines of upgrading, when/if you get to the point of the game where inventory slots are at a premium, keep in mind that this allows you to have wards+health in one slot, instead of having a slot reserved just for wards.