Pros:

"Q" and "W" have got CrowdControl
"Q" and "W" are Area of Efect
"E" aplies Magic Damage to basic atacks
"R" has got True Damage

Cons:

"Q" and "W" have cast time

Core Items:

Rod of Ages: This item Must be first complete item. If you have a lot of gold and Catalyst the Protector, it is a better option to buy Rod of Ages than Sheen.

Iceborn Gauntlet: This item is good in Team Fights or in 1v1 (good to chatch coward while damaging them).

Rabadon's Deathcap: Cho'Gath is also a mage and his abilities are very strong in 1v1 and also in Team Fights.

Rylai's Crystal Scepter: This item is perfect for a Tank and also to catch the cowards.

Sunfire Cape: This item is also perfect for a Tank because it damages nearby enemies.


Situational Items:

Banshee's Veil: It is another good item if the enemies have got a lot of AP.

Carnivore: Whenever Cho'Gath kills a unit, he recovers 20 - 71 (+2.83 each level) health and 3.5 - 7.75 mana (+0.25 each level).
Tips: If you are at low health and/or mana try to kill enemy minions.



Rupture: Ruptures the ground at target location. After a delay of 0.5 seconds, enemies are launched into the air for 1 second, dealt 80 / 135 / 190 / 245 / 305 (+100% of ability power) magic damage, and have their movement speed slowed by 60% for 1.5 seconds.
Tips: Try to not miss the target because it can be a fatal mistake and the result may be death or the enemy's escape.



Feral Scream: After .5 seconds, Cho'Gath screams in a cone in front of him, silencing enemies for 1.5 / 1.75 / 2 / 2.25 / 2.5 seconds and causing 75 / 125 / 175 / 225 / 275 (+70% of ability power) magic damage.
Tips: This ability is very good because the enemies within the area take damage but also silence.



Vorpal Spikes: Toggle: Cho'gath's physical attacks launch spikes which deal 20 / 35 / 50 / 65 / 80 (+30% of ability power) magic damage in a line in front of him. The width of these spikes increases the larger Cho'gath is.
Tips: This ability is good for damaging also the enemies behind the target which takes bonus magic damage.


Feast: After 0.5 seconds, Cho'Gath devours an enemy unit, dealing 300 / 475 / 650 (+70% of ability power) true damage (1,000 to minions) that ignores armor and magic resist. If the target is killed, Cho'Gath grows, gaining 90 / 120 / 150 extra health per stack (max 6 stacks). If the target was a minion or a monster, half the cooldown and mana cost are refunded. Each stack of Feast gives Cho'Gath 8.3 attack range, up to 50 bonus range. Cho'Gath loses half his stacks (rounded up) upon death.
Tips: The best option when getting this ability is to use on minions when aren't enemy champions to get some more hp.

Flash: If you need to escape over a wall or catch cowards.



Teleport: If you want to surprise enemies or gank them.



Ignite: If you want to secure/steal a kill.



Heal: Heal is always good.



Clarity: Mana is always important.



Ghost: If you want to escape or chatch cowards.



Smite: If you want Jungle Monsters' Buffs.



Clairvoyance: If two Wards aren't enough.



Exhaust: If you want to escape or chatch cowards.



Cleanse: Everyone hates Debuffs.



Barrier: A shield is always good.

I hope this guide helped you play Cho'Gath better.