You should always start with a Phantom dancer on master yi, It is very strong and works well with master yi's ultimate. After the phantom dancer you should either go with a infinity edge or a bloodthirster. The rest is optional. But if getting fed , Always buy a GA as u might die or simply buy a ryvenous hydra for extra damage and lifesteal

Pros
High Damage early game.
Fastest Champion in the game.
Best farmer in the game.
Good ultimate for either escaping or chasing.

Cons
Passive is only good late game.
Low health.
Has no CC

You should always last hit and try to get as high farm as possible. And since you can use your Q to farm , just use q on the enemy for a little harassment and farm. Its always good and always be careful for the ganks. They can destroy you.

You should always wait for someone to initiate a fight , you shall never initiate a fight. The best would be to wait a few seconds and the run in with a ultimate and clean up the kills. Since you have a low base HP you should allways wait for your team before entering fights.

Master yi works as a good jungler since hes Q can make a huge damage as AP yi. u often get a 800+ dmg to minions with your Q If u have 200-300 AP. He has a very good gank system thanks to hes Q wich is a very good gap closer. Hes ultimate works very good for ganking too since he gets a high attack and movements speed steroid when its used.