Yi is one of the best pushers is the game as well as a good carry. Yi has a good basic ability for builds with attack speed and with high crits he can do devastating dmg.

Abilities

Master Yi leaps across the battlefield with blinding speed, dealing magic damage to multiple units in his path with a chance to deal bonus magic damage to minions.

Master Yi rejuvinates his body by focus of mind, restoring health and increasing his armor and magic resistance for a short time.

Master Yi becomes skilled in the art of Wuju, passively increasing the power of his physical attacks. Activating Wuju Style doubles the passive bonus damage for a short time, but the passive bonus is temporarily removed after the effect wears off.

Master Yi moves with unparalleled agility, temporarily increasing Master Yi's movement and attack speeds as well as making him immune to all slowing effects. Additionally, killing a Champion refreshes all of Master Yi's cooldowns.

Pros

Pusher
High Attack speed
Easy escapes/Chase
High DMG
Farmer

Cons

Low health
Squishy early game
Easy to harass with ranged chars
Vulnerable to debuffs(not incuding slows)

Explanations

Runes,Masteries,and SS-

Runes- I put in a lot of crit dmg runes and crit chance runes because my yi is based off his beastly crit chance with it being 82.9% at lvl 18. These runes just "help" you get the 1k crits that you love oh so much........

Masteries- Well this one was pretty easy i gave 22 points into attack and 8 into utility the 8 in utility being used mostly for mana regen and overall mana because early game yi is very mana deprived........

Summoner Spells- Exhaust Exhaust is one of my favorite summoner spells. Especially when you have cripple in your mastery tree which lowers there armor as well as slow them. Its not really use full late game but early-mid game it has to be your best friend. Though sometimes i tend to use teleport instead of exhaust i still find exhaust more usefull. I guess its because i have almost always used it.
Cleanse Cleanse is extremely useful when fighting champs like sion,allistar,teemo, etc because all those champs can debuff you. Teemo for example can blind you and it can be a real pain if you cant hit him. Therefor cleanse is the Summoner Spell to use.

Items!~

Start off getting a Dorans blade Doran's Blade and potion. After a little while when you need to heal come back and get your second dorans blade Doran's Blade . Its an item i use early-mid game for your dmg and lifesteal but dont worry you sell it later. Though i say you sell these there is no excuse to not get BOTH dorans blades these are a must and theres no reason you would need to skip one of them.
-Mercury Treads Mercury's Treads- are the next item on the list. I personally get these because when im yi i always tend to get blinded,stuned,slowed (when finishers on CD), and it annoys me so i tend to go with these shoes that reduce the duration of those effects.
-Youmuus Ghostblade Youmuu's Ghostblade- This is an item that looks like it was personally made for master yi. it gives dmg armor pierce, cooldown reduction, crit chance, anything you could want as master yi. Also its active plus Master Yi's special can prove to be something extremely dangerous in the game
-Phantom Dance Phantom Dancer- well we all new that this had to be in here somewhere and well here it is. The Phantom dancer is an item that increses movement speed, gives crit chance, and attack speed. Its a lovely item that will pretty much make you an uber carry.
-Infinity Edge Infinity Edge- How can we have a crit dmg build without the infinity edge? well you cant. It makes crits do 250% instead of 200%. It also gives 80dmg and 20 crit chance.
-Madreds Bloodrazor Madred's Bloodrazor- Now i will say this. Madreds Bloodrazor gives the attack speed you always dreamed of. Not only that but it does magic dmg every attack for 4% of the targets Max health. So enemy tanks can give up.
-The Bloodthirster Bloodthirster- This is your last item. I put it in here for the sole purpose of its high DMG. The Bloodthirster gives 100dmg and it stacks when you kill something. You normally wont make it to this item, but if you do Master Yi is by far the best DPS char you can have.

FAQ!

Do you really need madreds bloodrazor?
No. Sometimes(depending on the situation) i get a Frozen Mallet Frozen Mallet instead of madreds. This works well for me and I think that its also a good choice. Though i still prefer Madreds.

Do You Really Need Exhaust when your Yi?
=S not really. Yi is one of the few chars in the league of legends who doesnt need an escaoe spell. Sometimes i see myself going with teleport. For ganking purposes.

Why mercury treads when you have Cleanse?
I use mercury treads+cleanse because cleanse has a ridiculously long cooldown and for when its on cooldown mercury treads help me a lot. Also sometimes i use Berserker Greaves Berserker's Greaves which gives 25% attack speed and if you want to get them they are extremely useful. I personally just like Mercury Treads

You said Yi was mana deprived but didnt give any mana items why?
Yi is mana deprived...Early on! Also i put points into mana regen and overall mana in my masteries to counteract the fact that yi has low mana.

I got the full build but i keep dieng right after getting a kill why?
This is because your not the only one with full build. Try trading out Madreds for frozen mallet Frozen Mallet for some hp, because late game when others have most of their builds Yi has some of the lower HP in the game. So the extra health that the Frozen mallet gives with the slow is a real plus if you seem to not be able to survive.

Gameplay!

Early game- Get your Dorans got to the top or bottom preferably with a ranged champion. Use exhaust to slow down the enemy champ and lower their defense. Use alpha strike and get first blood or at least an assist before minions even spawn. Use your potion and turret hug till you get its full heal. then go kill minions. When you get a chance go back and get your next dorans blade. Try to not be too aggressive after the early kill if you get to aggressive with your fighting you will die. Your health is one of the lowest in the league of legends early game so you do need to be careful. Also if your new to Yi I suggest getting heal for your first few matches.

Mid Game- Start ganking. Not hard. Go up behind the person in mid and use your finisher+exhaust and finish them off getting free kills like this gets you fed and quick. It will be even easier if you have teleport. Doing this is a lot easier said than done though. If you have champions like Vladamir in the Mid lane see if you can get your teammate to lure him close to the turret so his chances of getting away are slim to none.

Late game- You should have most of your build by now but i still dont advise going out alone mainly because by late game teams tend to stay together. go out with one or two of your teamates and just enjoy owning enemy champs. Though i say this watch out for enemy champs like sion, ashe, and malzahar who can stun, because this is one of Yi's biggest problems. When Yi is stuned late game it gives enemy champs a few second to drain your health, and health is something Yi does not have a lot with which is why i use cleanse for Yi and not ignite/flash.

HAVE FUN YOU GUYS!