Just a quick little guide for you.

Cho'gath is a great disruption/burst tank.
Rupture's knockup is good for initiating a teamfight if you land it on several targets.
Feral Scream is the second part of a basic combo with Rupture.
Knockup a team with rupture, then as soon as they're almost all done, let out your scream.
The entire enemy team is now slowed as they are down and are silenced for a second.

Your Vorpal Spikes is a great team damaging ability as you can only auto attack while
your spells are on cooldowns, and paired with a Rylai's crystal scepter, you forever
slow your enemy team down.

Feast, as you know, is what makes Cho'gath the giant he is known for.
While it does 1000 true damage to a monster and grants you a stack of fed when
you kill something with it, it is also great to burst down a squishy enemy.

Cho'gath's itemization may not seem as tanky as it looks,
but paired with 6 stacks of a level 6 feast, Cho'gath is a force
meant to soak up damage while dealing it.

Starting cho'gath off with a [*] Sapphire Crystal enables him to last longer
in lane with his mana costs as a single combo as cho can burn over half his
mana pool. Obtaining a [*] Catalyst the protector let's cho's large health and
mana pool regenerate on level up, and since Cho'gath is mainly played as a solo
top champion to rack up faster exp, he will be leveling up a lot.

Not much else to note, so I'll go quickly.

Rod of ages should be obtained before 20 minutes, or at the most 25, as it
takes 10 minutes for it to reach full potential in mana, health, and AP.

Rylai's crystal scepter also provides a large amount of health and AP, paired
with a great passive that enables cho'gath to slow enemies, allowing his team
to chase faster for kills.

Abyssal scepter reduces magic resist of all nearby enemies, and give you
magic resist and ability power at the same time, making you tankier and let's
your team do more damage as the aura debuffs them.

Frozen heart is by far one of the best tank items around, as it also has a
debuffing aura for the enemies that reduces their attack speed by 20%. It also
provides the second largest amount of armor from a single item and tons more mana,
along with a good amount of cooldown reduction to keep you actively bursting in
team fights.

Mercury treads are the boots you need as you need to be a disruptive tank, and not
being the one being constantly disrupted.

And finally, we have the deathcap. This will overall buff you damage as with Cho'gath's
natural tankiness and from some good hybrid items, you deal more damage while retaining
your badass-ness.

For now, you probably won't have to worry about runes.

Marks- Magic Penetration.
These increase the damage % your spells do, as all of Cho'gath's abilities are
true damage.

Seals- Flat Armor
In the beginning of the game, most champions have low base and low scaling armor.
These are recommended mainly because Cho'gath is a tanky and needs a lot of damage
mitigation for his massive health pool.

Glyphs- Scaling Magic Resistance
Almost every champions starts with 30 magic resist, so it's always going to be nice
to have more magic resist than the rest of the enemies, and since it isn't too needed
early game, these glyphs scale per level.

Quintessences- Flat Health -OR- Magic Penetration
Flat health quints are good simply because they give you a feel of better early game
tankiness.
More Magic penetration is good if you want more damage as opposed to ~100 health.

9 in to offense, which provides a minimal amount of ability power and
a small amount of Cooldown reduction, which is always a good thing, and
the last point into 10% magic penetration, which makes your spells even
stronger.

21 in to defense, providing some base armor and magic resist, scaling health
and flat health, plenty of damage mitigation, and the final point into
one of the best of the ultimate masteries.
Besides providing more health to you, you gain 10 tenacity, which
reduces the duration of any disables, such as stuns, slows, etc.

As I told you, you want to be the disruption tank, not the one being disrupted.

For your two summoner spells, you will want to take TELEPORT and FLASH.

RECOMMENDED

Teleport-
Teleport is necessary because Cho'gath is usually played as a solo top tank, and
as a solo top, you need to be dominating the lane most of the time, and so most
take teleport to maximize their farming and exp gain since it is a solo lane.

Flash-
Flash is also important in case you either need to escape or you need a gap closer.


Other good spells

Ghost-
Ghost is another good summoner spell in case you don't like using Flash.
If you can outrun your enemies with ghost, it's a good summoner spell to take.
If you can't outrun them with ghost, you probably need flash.

Exhaust-
Exhaust is good if you want more CC to add to your list of already good CC.
It's extremely good in team fights since Cho'gath's Rupture already provides
a slow if the enemy is knocked up, and paired with exhaust right after rupture,
it allows your team to catch up to that enemy quickly and stomp them.