Hello guys,
This is my first mobafire guide ever, plz comment if I make some mistakes. Master Yi is my main champ, I always pwn everything with him.

I advice 21/0/9. Just pick carry offense masteries, for damage and faster jungling. Also pick Swiftness to walk faster, Expanded Mind because your spells need lots of mana early game, and Runic Affinity to keep your buffs longer

Now its getting hard, you are a non-ranged carry. You can choose to get tanky and become a tank, I dont recommed this, because master yi can deal lots of damage as carry.
Open Vampiric Scepter, after going back first time pick Boots. Rush a Wriggle's Lantern to jungle faster. Now there are many things you can pick, First finish you boots, after that I should take a Zeal for the movement speed what makes ganking more easy. After that pick more carry items like Infinity Edge or Bloodthirster. Sell your boots when you got double Phantom Dancer, (with double dancer you have enough movement speed) and your wriggle's when you think you got enough lifesteal.

other good choises
Are people buying armor against ya? get armor penatraition like

1. Last Whisper
2. Youmuu's Ghostblade

Another good choise is Wit's End, because armor cant stop the magic damage.

Do your team realy needs a tank? get Frozen Mallet, you get more tanky and have some extra CC for ganking.

Dont you get much gold? rush a Avarice Blade, and later on buy Youmuu's Ghostblade.

not so good choises

1. Rabadon's Deathcap
2. Nashor's Tooth
3. Malady
4. Hextech Gunblade

Dont pick Ap or hybrit items, some people say an Ap yi is destroying, but they lie, only one of his ablitities does more damage with it. So dont pick these items.

Start with Alpha Strike, it deals some nice damage on neutral monsters. Try to max Wuju Style very fast, the passive gives you huge damage. I advice one point in Meditate early, so you can heal yourself if you need it and you dont have to buy health potions. As with all other ultimates max Highlander at lvl 6, 11 and 16.

Smite, just need it for jungling, take buffs, baron and dragon with it to secure them.
Ignite, its just my favourite spell, no one will escape with 10 health when ignite is on them.

other good choises
Flash, you can easy get close to your opponent and kill it more easy, but you already have alpha strike so it isnt realy needed.
Exhaust, master yi has no CC so exhaust can be an easy way to kill your opponent when ganking.

avoid
Ghost, your ult has almost the same movement speed, use it as ghost if you need it.
Clairvoyance, this is only for support just dont pick it.

Start at the blue take this one and go for wolfs. After it take ghouls and the two golems. Got some health left? pick red and recall, if you don't have health left, just go back. pick boots of speed (and after it maybe your red), then gank a lane. Just keep taking jungle camps and ganking, take much dragons (the global gold is 2 1/2 kills).

Ganking
Think what lane you have to gank, is Kassadin or Vladimir mid? don't gank them, they will easy escape with bloodpool or rift walk. Communicate with bot if the ward is still there and if they or you will destroy it. Wards are you biggest opponent, think of buying a oracle's elixer to destroy them and make it easy to gank.