CONGRATULATIONS!!! you've selected Lee Sin great choice... now to "Learn" him... Lee Sin isn't complex but is very easy to play wrong. He is not a tank, the closest he can be is a sort of meat sponge to soak up the damage. He is squishy when hes not hitting things, similar to an old Master Yi before malady was changed. End game assuming ur not CC'ed to death, you should survive all team fights.

Basically this build lets you take on whole teams if you start off good. (good means 4 kills+ in first 10 minutes with no deaths) In 3's the only challenge he has is rammus as he tends to kill himself before starks. In 5's just watch your not targeted first and you should come out on top.

APen to slice through tanks and any beefy dps. Crit so you dont have to buy crit items. Cooldown reduc so you can hit faster.

Dorans blade gives you a leg up in the beggining because his abilities scale so well with it at a low level. The boots are the clear choice as Lee needs attack speed over the other choices. The madreds is a key part of this, as end game with his passive you should have 2.2 attacks per second, making even the beefiest tanks crumble in your wake.

Ignite. Early kills. Gives AP.
Exhaust. Turns the tide as you're specced into it. Slows them down to catch them or to get away. The only offensive spell that can be used as defense as needed

As you will figure out after a few games, u miss the skill shot and they will get away. You just have to get used to it and its speed. One trick is to use safeguard on a teammate/minion near your target and then skill shot from there. Try to use your Dragons Rage to finish off the first target in a team fight and pop-up the rest of them. It is really helpful to your teams survival.

End Game: you should be running around with their entire team afraid of you. Regardless of how bad of a start you got. This fear will let you backdoor more easily as they wont know how many of them have to die to take you down.