So....Cho

You.... don't? He is a mage-tank after all. You know?

Cho'gath is basically an AP Fighter (a mix of Mage and Tank attributes).

You don't want to increase the damage his basic attacks deal; you want to increase the damage his abilities deal by building AP (in moderate amounts). His E is a spell which casts on every basic attack, dealing magic damage as it passes through the targets and a bit beyond.

Some AP items to build on Cho'gath include, but are not limited too: Generallly, items which provide AP and durability.

If you are looking for a decent utility build, I would recommend building something along the following lines: {Preferred Boots} [Not necessarily in that exact order]

Two Doran's Rings provide some early durability, damage and (most importantly) mana sustain. You'll be able to sell them later in the game after you have purchased a mana item, but these will be quite useful until that point.

Rylai's Crystal Scepter will make your abilities slow your target. This can make your Q simpler to lane (by using W first) and allow you to perma-slow targets (since E applies on-spell effects).

Frozen Heart provides a large amount of armor (to synergize with your passive sustain and ultimate's health), CDR (lowering your ability cooldowns so that you can provide CC more frequently and stack your ultimate faster), and mana (to sustain your ability usage). The attack speed slow aura is also valuable since you will typically be in the middle of a fight and it will affect most of the enemy team.

Spirit Visage provides a significant amount of health and MR, some CDR, and increases your passive's sustain.

Liandry's Torment will provide a percentage-health burn on your spells (including your E) which deals doubled damage to CC'ed targets (including those affected by Rylai's slow)

Zz'rot Portal provides some additional mobility around turrets (which can help your tower-dive or escape), a signficant amount of armor and MR (to synergize with your high health pool), and Voidlings (which can let you apply more pressure when splitpushing or stall lanes pushing into you).

Anyways, I hope that helps you somewhat.