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This is my Cho'gath build, which I honestly made throughout a match instead of sitting down and theory-crafting.

The difference with chogath compared to a lot of other champions is that cho'gath deals alot of damage even without offensive items, especially when his ultimate is the spell with the highest raw damage output in the game, which not to forget also deals true damage!

Pros
-Vorpal Spikes makes last hitting extremely easy, and also harrasses
-Carnivore(passive) makes health regeneration unnessessary
-rupture->vorpal spikes cleans minion waves in seconds until very late game
-Feast instagibs easily even without high ability power, due to the extreme raw damage
-Feast(passive) makes cho'gath able to tank successfully without much health items

Cons
-Rupture is easy to avoid
-Feast(passive) loses stacks when you die(WHEN you die)

- should be used for escaping or charging after someone to feast!(GREAT CHOICE!)
- saves you from CC's, is a must for most tanks, ALSO cho'gath.(There is no "no thanks" here, you TAKE it)
- If youre the kind of guy who escapes better with a little leap of faith, and same with feast, then pick flash instead of ghost(Question of preference :) )
- Possible choice if you love fortify, but flash/ghost and cleanse would be a better choice in my opinion, this counts for most other summoner spells too.

Whenever Cho'Gath kills a unit, he recovers 34 - 68 (+2.5 each level) health and 7-15.5 mana (+0.5 each level).

This spell is what fuels your ability to harass in the laning phase and also what carries you through the enemy harassment.
Ruptures the ground at target location, popping enemy units into the air, dealing damage and slowing them.

Your initiation spell, use it, and charge into the fray! Deals high amounts of damage even without AP, use it to harass in the laning phase. works well to catch up to runners when you chase, if you can aim it properly(takes alot of practice). Can lastly also be used to stop channeling spells like crowstorm
Cho'Gath unleashes a terrible scream at enemies in a cone, dealing magic damage and Silencing enemies for a few seconds.

Scared of some overfed caster? dont dare enter a fight with fiddles out there? wait for him to charge up, and then scream his head off, he will be crippled by the missing ult for a while. This spell should be used to follow up rupture normally, but if youre trying to hold a certain champion down, then wait for them to do whatever instead.
Cho'gath's attacks passively release deadly spikes, dealing damage to all enemy units in front of him.

Your farming ability, this attack is added to your auto attack, and it SCALES WITH AP, not only that, it also works with rylai's crystal scepter! so when you optain rylai's crystal scepter, your auto attacks will be upgraded to AoE slows!
Devours an enemy unit, dealing a true damage (1,000 to minions) that ignores armor and magic resist. If the target is killed, Cho'Gath grows, gaining maximum health (maximum 6 stacks). Cho'Gath loses half his stacks (rounded up) upon death.

This is your most important spell, always try to pick someone off with it in a teamfight, 300/550/800 true damage does do a difference, your preferred scenario in a teamfight is to have 6 feast stacks.

Skill Sequence
There is 2 difference ways you can go here, if you're gonna follow the guide, go: R>Q>E>W(Except for an early one at lvl 4). If you can't hit with rupture and don't want to practice, go: R>W>E>Q(except for an early one at lvl 4), this is not reccommended though!

Marks of Insight: Magic Penetration - will make your spells hit harder % wise! much better than AP or anything else, and marks primary attribute is ONLY offensive, so theres no better choice for cho'gath, in my opinion.
Seals of Resillience: Flat armor - will make you take little less damage, Manaregen flat/per level is a good other choice if you dont want armor, manaregen will make you able to harass more.
Glyphs of Warding: Flat magic resist - Will make you take less damage, Ability power flat/per level or CDR would be good other choices if you dont want magic resist.
Quintessences of Fortitude: Flat health - always a good choice for pretty much all champions, even cho'gath, 78 health is pretty neat, especially early game!

Not many masteries need explaination, the 9 in offense is to get spell penetration which will increase your magic damage output % wise. Ardor(defense tree - 16 mastery required) increases attackspeed, and ability power by a %, not much, but sometimes you have those games where you decide to buy rabadon's deathcap because you dont NEED this and that item to own. Strength of spirit will increase your health regen by some of your mana, its better than 1% dodge chance which you cant rely on AT ALL anyway.

Doran's Ring - Instead of dorans shield? Why? whenever you kill something, you get health back worth about 25 seconds of health regeneration of Dorans shield(only), and dorans ring will give you more manaregen, giving you more mana to harass(and farm) with.
Boots of Swiftness - you need the 3 movementspeed, ninja tabi isnt needed, youll get armor from other items, mercury treads doesnt matter, if you got stunned/slowed/anything'ed, the chase is lost anyway! sorcerer's shoes isnt needed, your most important damage is True damage anyway.
Banshee's Veil - You buy this early for the mana and health, and the magic resist will help you later on, the shield will block some cc too. Another reason is, it builds off of catalyst the protector, and you use that early to get mana and health back.
Sunfire Cape - Armor and health item, it also improves your farming abilities, and when chogath is most effective, hes in middle of the team fight, smashing people with vorpal spikes, and therefore also dealing sunfire cape damage.
Force of Nature - HIGH magic resist item, also gives health regen, and alot of it. With your natural high health, youll regen alot!
Guardian Angel - Gives alot of armor and decent amounts of magic resist, and also will make you revive after dying, making you nearly invincible.
Rylai's Crystal Scepter - Gives more health, and some ability power, but what we buy this for, is the slow effect, now when you use rupture, feral scream, even Feast and AUTO ATTACK(vorpal spikes), you slow the target(s).

It has to be added that Rylai's crystal scepter shouldn't nessessarily be bought as the last item, and can be bought anywhere after sunfire cape, depending on your needs.

Champion selection and until you've reached your tower/entering your lane.

Right as the game starts, buy aDoran's Ring, and go to your lane, preferably lane with a ranged, if a jungler is present on your team, YOU shall demand the solo lane!(unless you dont feel comfortable about it ofcourse :) ) If youre in a solo lane, and you know it in champion selection, pick teleport as a summoner spell instead of ghost, unless you really want ghost.

From here on the guide will be split in solo and lane partner laning phase

Early game/entered lane (solo laning)

DONT check the brush with your face, use rupture in it once or twice to try and make the enemies leave it, if they were there(doesnt matter, you check it without risking your face). when minions arrive, wait for them to reach your tower so you can last hit safely under the turret. Just play safely, and wait for the minions to reach the turret before attacking. Try to harass the enemy champions with Rupture.
(Ding level 6)
When you reach lvl 6, you should try to set up a kill as soon as possible, when you hit someone with rupture, who has gone below 300 health, charge forward, use ghost/flash if you have it, and feast them. At lvl 6, this often will come as a surprise for them because they dont usually realize it before its too late.
When you get 2350 gold, Recall and buyboots of swiftness andcatalyst the protector, try and make the jungler hold the turret for the time. Go back to your tower, try to set up a gank if possible, else just go normally back and hold it, not much more to soloing here. This part of the playstyle should keep going like this until laning phase ends, then go to a next step in the lane partner playstyle.

Early game/entered lane(lane partner laning)

When the minions enter the lane, check the brush with rupture and run in, if you hit someone youll be able to get a few auto attacks off on them(also your partner). Try not to push the lane too much, because thats what cho gath does TOO well, make sure to last hit as much as possible, to keep your mana up to harass, but always save some for a sudden kill.
(Ding level 6)
When you reach lvl 6, you should try to set up a kill as soon as possible, when you hit someone with rupture, who has gone below 300 health, charge forward, use ghost/flash if you have it, and feast them. At lvl 6, this often will come as a surprise for them because they dont usually realize it before its too late.
When you get 2350 gold, Recall and buyboots of swiftness andcatalyst the protector.

Mid game

From now on, you should be able to set up ganks in the other lanes(Make sure your tower is safe while doing this, a kill NEVER justifies a lost turret), try to do this as often as possible, finish kills with Feast so you feast up and give the team kills at the same time, try to avoid using feast on minions, its a BIG, BIG waste. Also, if you ever think of taking a jungle buff, try asking a nearby if they want it, you dont need it. Help them get it. As often as possible you should try to help people with buffs if they ask you.(this counts in late game aswell)
If you do it right(and your team for that matter), you should be nearly unkillable in this part of the game, so take advantage of it, GANK GANK GANK. You need your build up as much as possible before you enter late game.

Late Game

Late game usually starts about the time you start making your last to second last item, you can also feel it when you start feeling alot more squishy than earlier in the game.
You should try to stay with the team, only leaving them to clear BIG minion waves for minions, using a combo of QWautoattack.
Your initiation combo in team fights is: Q as many as possible, if you dont hit, you should delay your charge into the fray as much as possible to try and do it again, dont mind if someone calls you a noob for " not initiating" because he got too impatient, its his fault.(remember, the tanks job: initiating, the DPS job: WAIT for the tank to initiate, NOT initiate if the tank doesnt seem to do it within 5 seconds). When you hit successfully with Q, run in, and hit as many as possible with feral scream, and then auto attack and cast spells as they come off cooldown, and be ready to Feast someone when they fall below the threshold(threshold= the amount of damage feast deals, Feast deals 100% damage to all, true damage).

Other information

Most of this playstyle part is just tips, dont follow them step by step, some things is up for you to decide, if the other team has no Casters, change your banshees for rod of ages, and buy something like warmogs armor or thornmail instead of force of nature. if the other team is caster heavy, you should buy force of nature after banshees, instead of buying sunfire cape. if youre doing EXTREMELY well and dont need more armor or magic resist, buy rylai's crystal scepter earlier.

This is it for my guide, please put constructive criticism, and dont just down vote it because "my build is better" or without trying it out, this counts for up voting too ofcourse :) Try before voting pretty much. and if you down vote, please say why, dont just come in and say "bad" or something... :( every time you do that, I die abit inside :'(

I spent a lot of time on this!

Hope you enjoyed :)

- Frizer