everything in this guide was chosen because it allows cho'gath to stay on a lane and keep pushing. This is tuned to make his passive work better. also, the mana stuff is to make sure that cho has enough mana in the begining and to ensure that he never runs out.

lets start with the dorans ring and chalice. the ring will give you a small boost so that you wont have to recall as much. you WILL need to SELL this item later for the other items. the chalice ensure you dont run out of mana (unless you somehow over spam your moves). you will also sell this item.

POTIONS. depending how you play you might need more potions. buy them as you need but you i recomend buyinh 2 potions withing the first 10 minutes.

most of the items beyond this dont need much explanation. sell the dorans ring when you reach philosopher stone. sell the chalice when you reach shurelya's reverie. you can buy phantom dancer and rylai's crystal scepter in anyorder you like, but i do prefer you getting phantom first.

in fighting, use rupture first. it gives you an almost guaranteed hit with your ultimate. the use feral scream once he hits the ground to make the most of his silencing effect.
the reason i would max rupture first is cause it stuns him for 1 second, it has a good AOE, and it does good damage starting out. also you can use it to escape. ex. before you use flash, activate your rupture where you where standing. the flash blocks out some of the green ring that shows up. now you have a 1 second advantage.