OMG YI N00B!!! this is what goes through most peoples heads when they see a Yi in the loading screen but despite what people think he's actually a rather difficult champion sometimes (mostly because these same people almost always focus him)
also please note this build is mostly here to help you with skill, mastery, and rune choices if you don't like the way i build yi then you don't have to use this build this is just the way i like him (note: currently undergoing a rework of the whole build my words will disagree with my choices for a little while)

Pros:
Fun to play
very fast
excellent farmer
HE'S A STINKIN JEDI!!!
Cons:
Focused.....ALOT!!!
sometimes hard to get build off the ground

A simple 22/0/8 straight forward and to the point and just the best way to go for DPS champs
and for those people who think that is a bad idea http://www.leagueoflegends.com/board/showthread.php?t=219077 check this out first......please

YAY XD the alacrity runes explain themselves but the mana regen is because Yi does struggle with mana problems early to mid game and the cooldown runes are just so you can use alpha strike more often XD

as to farming i suggest that unless your playing against idiots stay just in range of you minions to scoop up the exp and occasionally go in and get last hits but you should try and stay under your turret as much as possible until lvl 4 then you can do loads more damage and should be able to get a kill or two

for ranked your job is to help in teamfights and back door the other team with this build you need 2 minions at full health and 10 seconds alone with the turret and it's gone so good luck :)

please comment on it :)