Well I just played my fifth game , io mean my fifth lost with him..
NO idea how people carry that much with him O.O

In teamfight , I get CC then die in 1sec WOW ?
tried 3 build so far.

one straight Life steal , was **** cuz no dmg out push/AS
one doran shield merc boots benshee , and still getting owned so fast (had 3mage)
dor blade , berz boots , IE , youmuu ... trash

so
wtf am i doing wrong O.o...

tried 21-9-0 9-21-0 9-0-21...
always went ghost exhaust or ghost cleanse so far

you dont ever engage in teamfight a yi, split push 24/7

Yup team fight if you get cc, you die. Either enter a team fight last if your team is still alive or go push another lane.

Yi is not meant to take damage in a team fight, that's what the tanks are for. I'd wait a moment and then try to Alpha Strike in and then use your remaining skills. You can take out their squishies really fast so figure out who will die the quickest and go after them. Try to collaborate with your team and agree on who you'll focus on. I had a really annoying game as a Veigar because my team kept splitting up every single teamfight and their Yi would have a straight path to me.

So that's what I would suggest your plan would be: let someone else engage first, then you can join in on the fun and take out their weakest champions. Also remember in smaller teamfight situations that if there are a couple minions in front of their champion, have Alpha Strike hit those minions in front first, that way you'll hopefully end up there rather than aiming at their champion and ending your attack right next to them.

I'm still trying to find a Yi build that I like but I'd suggest you try going AP or hybrid in a co-op vs AI game so that you have more mana to spend practicing your attacks, as well as having more AP to help your heal skill be more effective. I'm not saying that you shouldn't be an AD Yi, but an AD build usually leaves you low on mana.

All of the above people are right when they say you shouldn't be engaging in team fights. However, that doesn't mean that you CAN'T. Let me give you some advice that works for me:

Start off with vampire scepter, and build it into a starks after finishing your boots. Grab alpha strike first, but after that focus on your E and max it out. You won't need to worry about attack damage midgame when you have a high leveled E, so focus on attack speed. Get a ghostblade or a phantom dancer. From there a bloodthirster and Infinity Edge come next. Your sixth item should be a phantom dancer, or else another bloodthirster while you sell your boots for another phantom dancer later on. This will not make you invincible in team fights, but will make you nearly unbeatable if you can pick them off one at a time. You can do this by either entering a fight late and picking off the runners, or split laning and waiting for them to come to you. The most satifying thing is to watch a self-sustaining yi get a quad or pentakill and come out full health.

I hope this has helped you.

Be weary of teams full of ignite though. If the opposing team has 4+ ignites, it's probably more cost efficient to not get lifesteal and try to get armor pen, crit, ad/as.

I don't entirely support this build, but as an alternative, build tank/AP yi and max out his W. You can literally stand there and channel your W and take almost no damage if you build right so long as no more than 2-3 focus you at a time. I don't highly recommend it, as its highly touch and go and depends on who you're facing, but if I find my team lacks in tanks and im using yi, I might use this for the fun of it.

Ok, here's what you do:

So the team fight is about to start, both teams are hovering near each other. Ideally, you are in another lane farming/pushing. As soon as you wipe the current enemy creep wave out, head towards the fight. Make sure theres no creeps left so no enemies that are paying attention can know for sure you're coming. Hover out of range watching the battle. Do NOT join in to the battle itself. As soon as their squishiest player (prolly a mage, possibly a carry) is around ~50% hp, alpha strike to him. the instant alpha strike ends, you'll be next to him (assuming {LULZ} that you didn't kill him already.) As soon as you land, activate wuju and highlander and right click that enemy. He should die almost immediately. And your cooldowns get refreshed. Alpha strike to next squishiest. Rinse and repeat till GG.

Alternately, when the team fight starts, start blowing through your mana to push a different lane into their base. <-This is what most guides/Yi players will tell you to do. Really depends on the situation, location, players, and champions.

As far as not dying: ignore every other guide out there, and start off with a regrowth pendant and a health (or mana if you want, but you shouldn't need it) potion. If the enemies are focusing you even a little bit, being aggressive, ganking you, etc., build warmogs. Just do it, you'll thank me. Otherwise, sell it later when you don't need it.

uh get zeal, valor, and a bf sword

well you should go jungle with him:
Item build:
Rejuvenation Bead+6 hp pots
start at mini golems with wuju style and open with smite then kill both of the golems then head up to wraiths and if the blue wraith didnt die becaus of your alpha activate wuju style
then kill the wolves (always use hp pot) the kill the big golem with smite and wuju+alpha combo...
go back to base buy yourself some pair of boots then go mini golems again(becuase ur smite is on cd)
after u killed the mini golems go to red buff and kill it with smite ofc
go gank...
well that is it all u need to do is this ^
you item build:
wriggles
berserkers greaves
avarice blade(upgrade to ghostblad after u got trinity force)
trinity force
atmas/warmogs
warmogs/atmas
in teamfights ofc always focus the carries always activate ur ulti so you can kill the carry and then get the cd reduce to kill the other guys...
masteries: well i go 0/16/14 (get the hp and 2 points on the speed buff on the utility tree)
summoner spells: smite& ghost,exhaust,cleanse
R>E>Q/W>W/Q

€:if you dont go jungle with yi then you should go ap yi LOL