Focus upgrade Vorpal Spikes [ E ] skill to increase farming and to make it easier. For now other skills will be just engage or escape utilities.

Cho'gath passive is recovery life from kill minions what can be potentialized by Feast so cho'gath can stay more time in lane, increasing your farm.

First time going shopping UNLESS you have a HUGE advantage over your enemy DON'T buy Rod of Ages first buy dead man's plate if you fighting and AD champion or Spirit Visage against AP enemy. If your enemy have a stun or prision skill maybe is a good choice to buy Banshee's Veil instead of Spirit Visage. This way you can stay more time farmining.

Your ult Feast have 6 stacks that make your range, life and size a lot bigger! To stack this is very simple kill a target and you gain 1 stack if this target is a minion or monster half cooldown and mana is refunded! don't forget against minions and monster it deals 1000 damage making farming easier.

I guess you understand if not I'll be clear FARM IS EVERYTHING!!!

This way you surely will have advantage in MID and LATE game!

Cho'gath main skill is Rupture [ Q ] this is how you engage a fight, escape or control a group but unfurtunatly it is a kind of easy to escape.

So first of all to make sure your Rupture [ Q ] will be sucesseful you must silence your enemy using Feral Scream [ W ] skill making sure he can't use a skill or flash.

When your enemy is low life you can ult Feast him, your ult deals true damage and it can be a bad surprise to a distracted enemy.

Game highlights and commented to make sure you understand concepts of what to do and not to do:
Play Video