General advice
Here I will give you some advice for each Cho'Gath Ability

[Q] Rupture:
Predict the enemy movement to increase Q Hit-Rate.
Combo your Q with CC-Abilities from your own Team (For Example: Leona Q)
If the enemy has a lot of mobilty (For Example: Irelia Q), try using ur W before your Q, so she gets silenced and can't Q out of your Q

[W]Feral Scream:
Don't waste this Ability! Use it whenever you need to shutdown an important target. It works best against Bruisers/Assassins, keep it for them.
Don't overuse it early game, the mana-costs are to high.

[E]Vorpal Spikes:
Use it whenever it's off-cooldown when you trade with the enemy. Lvl 1 you want to push the wave with it aswell.
Remember that it is an Auto Attack Reset and it has a small slow, you can use this slow to hit your Q easier!

[R] Feast:
Do not use your Ult before any objective (Herald, Drake, Baron)
If you are sure, you will not kill the enemy champ in the next 20 seconds (when your ult is up) use it on a minion, if you know you can make a play, keep it.