Master Yi is a highly underrated champ. His laning phase is incredible easy against basicaly all champions through his strong poke and his Meditate.
When played correctly he can deal crazy damage through the cooldown reset of his ult. (okay that means kill stealling ;-) )
He is my favorite champ and i like to show you how i play him and maybe get some feedback.
I have played over 200 games with him alone, and he carried me to an 1400 elo. (80% win-rate)
Nevertheless i decided that i am by far not suit for ranked because i played all my normals mid and the other rolls are yet to be mastered.(You can still trust me on Ap Yi)

Take your time read the guide and try it out... you'll be having fun destroying enemys garenteed.

Please dont pay attetion on grammer or spelling misstakes because i didnt either^^

Take some Magic Penetration and full on ap Runes.
So you will get the most out of your Alpha Strike in early Game.
Skalling ap is okay. You dont need so much mana early if you do buy saphire cristal.
Magic res. is **** because you can meditate and survive the enemys burst pretty well. Katarina 's ult totaly wasted when your standing next to her meditating...
The magic pen. + boots give you true damage against ap/ad-carries.
Not much more to say at this point.

21/0/9
The only thing i want to say here is that Master Yi can take down turrents without minions late game pretty fast thats why u take 1 Point in the Turrent-thingy

Starting Items:
Start with boots 3 potions. When there is no jungler on the enemy team and your laning against a champ where there is not a lot to doge take saphire cristal and 2 potions.

First Buy:
Buy Dorans. One or two. A blasting wand. Maybe a Saphire cristal for more sustain.

Core Items:
Sorcerer's Shoes. Rabadon's Deathcap. Lich Bane:
Those are your core items. Lich Bane is the item u will need the most. It will ad a somewhat sustained damage to your Alpha Strike Nuke. It has a 2 sec. cooldown. Rabadons is just for maxing your damage on Alpha Strike.

The rest of the Items:
Here you build whatever is needed. Void Staff is someting to consider after your core items. Rylai's Crystal Cepter is good cause it grants you Health and more Damage. Zohnjas Hourglass is good for damage and when all of your spells are on cooldown to cover some time.

Get Meditate on lvl 4. Before you can Meditate you can just use your Potions plus the lvl 2 Alpha Strike will keep the enemy even lower. Skill one Point in your e and activate while alpha striking your target. It gives decent damage in early game. In mid and late game this ability is used mainly to activate your lich bane.

Aggresive Play:

In early Game you last hit minions and whenever there are only 3 or less left and the enemy champ is arround them use Alpha Strike on the minions.
Whenever your passive is ready try to land an alpha strike and activate your e while jumping. This does insane damage.
Keep the enemy low.

Passive Play:

Do last hits. Dont spam Alpha Strike. Try avoiding taking damage. Meditate and use your health pot.

When to play aggresive and passive:

A good jungler will gank you pretty soon after you start pushing the wave, and pre 6 you dont have you an escape plus your Meditate is not strong enough to survive a gank.
I still play aggresive just because my Flash can save me, and jungler usually dont gank before they finished their first route through the jungle. Meaning you can play aggresive up to minute 3, then stay back a little until you skilled Highlander.

Something you can try is push the enemy to your tower:

- Creepwave 1 reaching mid at aproximatly 1:55. Clear this wave fast.
- You Creepwave 1 will get in tower range.
- Your Creepwave 2 will get in tower range.
- Your first two creepwave will have fallen to the enemy turrent at 2:45
- Enemy Creepwave 2 will still be pretty healthy and push you

Positive Sideeffects:
- Enemy lost creeps to tower
- Your pushed
- You had the chance to harass

Now you have your ult.
If your enemy hasnt left the lane and is low hugging his tower you can try to get him.
Activate your ult if necessary flash towards him. Alpha Strike Ignite one hit and leave.
Heal up and start farming with Alpha Strike this will push the lane. You opponent will lose minions to the tower and you have the chance to roam. (leave your lane often get kills and then go back to your lane and push like crazy, when pushed roam again)

What to look for when Roaming??

- Whenever on another lane some enemy is low this is the perfect setup.

- When for example bot gets ganked and they tower dive your team go immediatly there. Chances are good that one enemy will be very low and you can get them with one alpha Strike. With your ult you can alpha strike again without cooldown. I have gotten 1vs3 Tripple Kills a lot of Times like that.
Kill your tower as soon as posible.

- Whenever you feel like it...

Team Fights

Be the last one to engage. Wait till some one is low enough to fall your alpha strike. With your ult your cooldowns will reset and you can alpha again immediatly. If the enemy team still has hard cc left you shouldnt try to meditate in the fight. Leave the fight and come back full heath.

Your job in team fights is still the same.
But with your Lich Bane you can take one the ad carry or another squishy guy by yourself. Highlander + Alpha Strike + Lich Bane + ( ignight ) . Repeat
In late game you should be able to solo a tower without minions. The Key is the passive Effect of your lich Bane:
Ult-hit-wait 2 sec-e-hit-alphastrike minons-hit-meditate up-hit and so on.

Dont engage on a 1vs1 without having poked the enemy.
If u have to. Alpha strike-Lichbane-e-lichbane-w-alpha strike-lichbane
if that doesnt work r-run^^