Hello Mobafire! This is my first guide, so leave me some feedback if you enjoy it or see ways for me to improve it. I decided to make a Cho'Gath build guide because I have been doing very well with Cho'Gath with other people's builds but wanted to branch off and make my own. Why do I think I'm qualified to make this guide? I think my match history speaks for itself:

Please, don't give me too hard of a time, again, this is my first attempt at making a build and it might need some obvious work that I somehow missed in the various "how to make a guide" guides. If so, comment and tell me what it is!

If you haven't already, read Egypsian_Lover's very excellent Karma guide, at the very least the General Game Basics section. Even if you've been playing a while, it will make you a better player with any champ.

Greater Mark of Magic Penetration 9 magic penetration from level 1 gives you a pretty significant damage boost on your Vorpal Spikes early game.

Greater Seal of Armor Flat armor is a cheap, good, tankiness rune.

Greater Glyph of Magic Resist MR is also a cheap, good, tankiness rune.

Greater Quintessence of Ability Power 15 AP may not seem like much, but it that's an extra 15% damage on your level one Rupture.

These masteries allow Cho'Gath to be incredibly tanky while still dishing out some solid damage.

These two items are essential to health-based tanks. Merc Treads because your tank can't bother anybody when they can't catch/attack them, and Warmog's because it is quite simply the best health item.
Why start with Doran's Ring? Because it gives you an extra little bit of tankiness until very late game and because the AP and mana regen are very useful for early game success. Take this if you have a solo lane.

Why start with Doran's Shield? Because it's an excellent starting item for tanks in general, buy this if you have a lane partner who can dish out the damage while you take the hits for them.
After a while, you really need to go back for mana. If you kill an opposing champ, go back right then. Otherwise, stay out until they go back to base or until you are waiting on your mana regen to be able to do damage.
When you go back, you need boots. At the very least, grab Boots, if you can, get Mercury's Treads because the tenacity will save your voidian *** in a team fight and the magic resistance will save you from the enemy caster mid.
You want to get Warmog's Armor as fast as you can. If by some black magic you have 3000 gold on your second trip back, add mtma3 in game, I'm putting you on my ranked team. If you don't, either get Giant's Belt or the other two subitems, depending on how much cash you have.
Now you're got some cash, a few kills, and the ganking stage has begun. You need more tankiness to survive, more AP to kill or severely injure your opponents, but damn could you use some more mana. Now let's think, what item solves all of these problems and in addition gets better as the match progresses? That's right, Rod of Ages! After that, it's full on supertank time. Get Frozen Heart. The mana is a godsend, as is the monstrous armor. The CDR lets you feast on squishies 20% more often while the AD carry is dealing damage 20% slower thanks to the other passive.
The next items you want are either Force of Nature or Thornmail, followed by rylia's crystal scepter. You want Rylia's for the AP boost, the extra tankiness, and to boost the slow of your Rupture. You want Force of Nature against heavy AP teams for the massive Magic Resist, not to mention the excellent health regen boost; with your huge amount of health you will have +22 health/second just from the % health restore. However, if you are up against a lot of AD, buy a Thornmail instead, the huge armor boost, while amazing, is nothing compared to it's amazing passive: making AD carries commit suicide.

You want to take Rupture as your first ability. It has the largest range of your abilities, allowing you to hit long-range champs and fearful, turret hugging opponents alike. It also knocks them up and slows them down at the same time, allowing you to go in for the kill with Feral Scream and after you get Feast, to eat their puny little bodies.

You are the tank. You Exhaust the AD Carry.

Your other spell should be an escape mechanism. Teleport is really useful throughout the game, as it lets you provide assistance to your teammates when your lane opponents go mia and turn up on the other side of the map to gank your carry. Teleport can also help you get back into the action after you go back to base to heal.

Grab Rupture and your Doran's Ring/ Doran's Shield and go bot with your team's ranged ad carry, or solo top if both teams have a jungler. There are a few different posible types of champs you may face. You will probably find either a carry and a support, a single carry (solo top), or a carry and a tank. If you're up against a support, decide with your carry if you want to take out the support first to remove the carry's healing, or the carry to take the pressure off of bot lane. If you're up against a carry and a tank, ALWAYS focus the carry, focusing the tank is exactly what he wants you to do because you'll use all your abilities and mana on him and not have enough left over to slaughter the carry. Don't do that. Focus the carry, then kill the tank on your own good time.

Cho'Gath jungle is a solid choice, but I warn you, DO NOT ATTEMPT unless you can hit your Rupture 75% of the time or better! As his main CC, it's essential that you be able to hit it as much as possible.

My suggested Jungle Cho'Gath build would be something along these lines:
Jungle route:
Ancient Golem wolves wraiths lizard elder gank.

Cho'Gath is a very solid tank who can be played in a lot of different ways. Personally I love being able to sustain and deal a ton of damage, and having 6k+ health, a lot of armor and MR, and 750 instant targeted true damage every 48 seconds does that very well. I hope you've enjoyed my guide, please comment and vote!

2/13/2012 - Created guide
3/4/2012 - Finished my rune page, added my experience with how much of a difference they make
3/5/2012 - Added change log
3/5/2012 - The stupid editor glitched when publishing, and I now have to rewrite the rune section, this shouldn't take too long
3/6/2012 - All new runes, refresh to the items section, added core build and another starting item, rewrote Summoner Spells section, Playstyle section, and the Summary, changed skill sequence as I have gotten better at landing Rupture
3/12/2012 -

Re-enabled require comment to vote cause all you ***** downvoters don't have the balls to show your faces cause you can't give a legit reason for downvoting. MobaFire, please don't force that option off at 20 votes. It's the only thing that keeps the system honest.

3/12/2012 - Cho'Gath is now chargin' his lazor in the intro.
7/20/2012 - Added Jungle section, more changes soon!
10/1/2012 - Changed quints to something more useful on a champion with AP scaling true damage.
1/20/2013 - Starting to update for S3, still need to edit items section.