Hello and welcome to my Zilean Guide, it's my first guide and this is what I have been playing Zilean with succes in Ranked, hope you like it and get you started at leat with this great champion :D

We get Q first for max harras early game witch should make you get a kill or 2 for your adc, we always get R when we can, Then we max E for CC and mid game for ward placing on yourself, and last W.

Ok i'll explain why the items list order and how to change it depending on hows the game going
First we start off with Amplifying Tomeand choose between getting a ward or 2 pots, then we get Kage's Lucky Pick pick for gold generating, and then we have 2 options rush Ruby Sightstone or rush Rod of Ages.
By the time you got the 2 of them you should be in mid game around 15-25 mins, since there its going to be some roaming for you to put wards on, you should now buy Ionian Boots of Lucidity with should put your cd on 25%.
Then we got another 2 options get Morellonom and get 45cd which we exceed by 5% or get Twin Shadows for a more supportive role, After getting all this items you should be around 35-45 mins in the game and here comes late game.
Get for more surviability Rylai's Crystal Scepter or Zhonya's Hourglass since you may start getting first killed because of your ult in teamfights, if not get Rabadon's Deathcap first.