I will show you how you can pwn some noobs and pros with Cho'Gath. With this build, I got a score of 18/0/2 first time in PvP, so just good luck and have fun. :)

You need the health runes to have the most health possible, if you do this, you can have almost 7k health. You can also use the defense runes, but i don't recommend them. If you have lots of health, you will be almost invincible with the help of Thornmail and 300 defense.

Tanks need defense things so just get all the health and defense masteries until you get the last defense mastery.

You need defense and health items so that you can be a good tank, and with these items, you will have lots of them.

I recommend Fortify and Teleport, and if you can, try to convince your teammates to use thoes spells too cuz its easy for one of you to use Fortify, then the other one if it is needed and Teleport so that you can go to places faster. You can also use Ghost, but then you need to remove Fortify from masteries and add Ghost. Ghost is for running from a gank.