For stating items you can ether go Doran's Blade or Boots or Vampiric Scepter i usually take Vampiric Scepter so i don't need to take it later and it is gives me tone of hp tacks to Wuju Style.

For the beginner it is recommended Doran's Blade

Boots is recommended if there is enemy like Swain whit his trap ability (w) and Amumu or Cho'Gath whit slows that you can run out of the way of.

Vampiric Scepter is a lite more for the advanced user or the ones that knows how to play Master Yi for it is amazing when you activate your (E) and get in to get first blood :D it is also very suited whit Phantom Dancer for you attack more and crit more often = more hp back

but after that it is Phantom to Bloodthirster that is the way to go.

At this point it is time to chose if you are going for The Black Cleaver or Last Whisper this is an easy trick or a very useful tips i got to what to go for in this situation. if the enemy team got more then 125 armor you go for Last Whisper if less then it is Black Cleaver, Why you my ask well Black Cleaver does a stack for every hit it does it removes 15 armor (stacks 3 times) that is will be enuf for someone that has less then 125 armor for then you have removed 45 armor from him, but if he/she got more then 125 armor you damage will just disappear, that's when Last Whisper come in plays when it removes/ignore 40% of the enemy armor so if the enemy camp got for example 200 armor it will ignore 40% of 200 = 80 armor against Black Cleaver 45 it speck for its self why it is good, but then you my ask why at 125? well at 125 it is 40% of 125 = 50 that is 5 more armor penetration better then Black Cleaver which makes Last Whisper the better choice.

So to make it simple

less then 125 armor Black Cleaver

More then 125 armor Last Whisper

For last item it is more of the choice do you want to survive or slow or whatever i recommend Frozen Mallet that will give you some survivability and lite more damage and an always on hit slow for gradient kill.
Or if you get focus alot it is Guardian Angel witch will give you some magic resist and armor and Revives your Champion upon death, restoring 750 Health and 375 Mana that will get you back to life fast and keep on killing ;)

If it is an AP heavy team go for a Banshee's Veil.

If it is an AD heavy team go for a Frozen Heart or a Thornmail