Cho`Gath
Passive: 
Carnivore
Whenever Cho'Gath kills a unit, he recovers 34 - 68 (+2.5 each level) health and 7-15.5 mana (+0.5 each level).
Explanation:
One of the best laning sustainable abilities in the game. It keeps you at a safe HP when you last hit creeps and gives you back little mana when you decide to harass with W. Without this passive, Cho'gath would have an extremely hard time laning.
Q: 
Rupture
Ruptures the ground at target location. After a short delay, 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 3 seconds.
Cooldown 10seconds
Cost 80 / 90 / 100 / 110 / 120mana
Range 950
Explanation:
This is an important spell on Cho'gath. You use this spell to stun runners, when you're running, or when someone is stunning down a teammate in a fight (ex. WW, malzahar). And when a fight is about to go down somewhere (ex. at dragon), don't waste rupture for the extra damage, whereas when you miss it will be a huge waste because you won't have it for the next 10 seconds, which could determine who wins the fight. This skill seems unreliable for people at times, because they think its all about skill shots when actually... its not too hard to figure out. Use this skill to follow up with your other teammates stun/snare to guarantee you a stun (ex. taric stun, lux snare, etc.), and you will hit almost every rupture. Also, using this ability in narrow passages will guarantee you a stun as well (use rupture ahead of them, and if they want to juke it they'll have to walk back to you or stop moving.)
Don't use this ability too much in laning phase, because the person you're going against will almost juke it 100% just because its so easy to. Use it when you think its right, but don't use it to try to skill shot them and push your creeps too much by accident. Its a waste of mana to spam in lane, whereas Feral Scream (E) is a lot more reliable to use as a poking ability. It takes practice to master this ability, and if you can make all of your stuns in team fights, you'll be a huge contribution to the team.
W: 
Feral Scream
Cho'Gath screams in a cone in front of him, silencing enemies for 2 / 2.5 / 3 / 3.5 / 4 second(s) and causing 75 / 125 / 175 / 225 / 275 (+70% of ability power) magic damage.
Cooldown 16seconds
Cost 70 / 80 / 90 / 100 / 110mana
Range 300
Explanation:
This spell is very nice in almost all situations. It makes clearing waves of creep easy (in combination of rupture), poking in laning phase extremely effectively, and silencing enemies. This spell is pretty simple to use. Once again, don't waste it on enemies before a team fight who arn't even doing anything and you're only using it to do damage - Thats a big nono. You need to use this spell to support your teammates. Like rupture, use it to interrupt enemy spells (WW, malzahar ult) and it should be a lot easier and faster than rupture.
In addition, this spell's range is actually further than it appears. When you see the cone in which you're about to scream, champions who walk out of the area will still be silenced if you use it quickly enough. Stack this move, and then you'll be doing a lot of damage for a tank and silence the enemy for a max of 4 seconds!
E: 
Vorpal Spikes
Toggle: Cho'gath's physical attacks launch spikes which deal 20 / 35 / 50 / 65 / 80 (+30% of ability power) magic damage to a line in front of him.
Range 40
Explanation:
Vorpal spikes is a great farming ability. It helps with killing champions as well, but its hardly noticeable. It makes your last hitting a lot easier than when you don't have it, but toggle it off if you feel like you shouldn't be pushing and you want to keep the creeps on your side. But usually in a game, always keep it on in fights because it provides extra damage. And importantly, be sure to toggle this ability off when you're taking a tower, because your vorpal spikes will still hit your enemies when you're on the tower then the tower will start attacking you.
R: 
Feast
Deals 300 / 475 / 650 (+70% of ability power) damage to target champion, or 1000 damage to target minion (ignores resists).
If the target is killed by Feast, Cho'Gath grows, gaining 90 / 120 / 150 maximum health (maximum 6 stacks). Cho'Gath loses up to half (rounded up) stacks upon death.
Cooldown 60seconds
Cost 100mana
Range 100
Explanation:
Feast can be used in 3 ways:
1. Feast creeps to build up your stacks but don't get too ahead of yourself and feast a creep when a team fight is imminent.
2. Feast can also be used as a smite. It does 1000 true magic damage at all levels which is more than smite, and in combination of your jungler and your feast will guarantee you dragon/baron almost all the time (Cho'gath and jungle Nunu is godlike).
3. Feasting on a champion. Remember that Feast deals TRUE damage, so watch the enemies health before feasting because that can determine if they get away or not, unless feasting earlier will help getting the kill for someone else. Feast cannot go through shields (banshee's, sivir's spellshield, or poppy's ulti) so be careful who you use it on.
In a teamfight, its best to use your feast earlier than intended (especially if you have 6 stacks already) on carries which will make them panic because you just took a chunk of their health with one spell. For example, a farmed Cho'gath who ghosts then feasts an Ashe will drop her very low and it will make her hesitant on what she should do next, and it will allow your teammates to make quicker work of her. This ability is so much damage for a tank. Its like a Garen ulti without the limits of them being low.
NOTE: Explanations taken from Westrice guide. I just can`t say it better. Credits for this chapter goes to him.
Skills
Q W E W W R W E W E R E Q E Q R Q Q
Explanation:
prioritize R > W > E > Q
-Your ruptures (Q) main role is to stun, not to damage. So only get one point in it for now. If you stack this, the mana cost increases. Keep that in mind.
-Feral scream (W) is your primary poking ability and silence. Stacking this will increase its damage significantly and silence them longer (and its not a hard skill shot like rupture). Stack this ability.
-Vorpal spikes (E) is mainly used for farming, and since Cho'gath is a strong farmer, prioritize this ability after W so you can free farm with ease. Poking enemy champions with this ability does a lot of damage as well, so getting free hits off on them with this ability on will do nice extra damage. Plus this ability uses no mana.
-Take feast whenever it is up, as when you level it up, the true damage increases (300/475/650)
Build Guide
Discussion
Flash + 
Ignite and as mid Cho`gath rly nothing else. Ignite is so good with your ulti that late game you deal arround 2k+ true dmg. HOLY **** !!!!!!
Greater Glyph of Scaling Magic Resist kicks in. I highly recommend the quints listed in the guide for mid lane. As Cho`Gath you want to stay on your lane and farm as much as possible. MR quints + 
Cassiopeia.
Greater Quintessence of Ability Power. This +15 ap will give you the ability to trade with the bruisers.
Greater Quintessence of Movement Speed and max you "e" first.
Boots of Speed + 3 pots is a must starter for any mid lane imo. Without it you may not be able to trade with enemy ap + you are weaker to gangs from the jungle.
Catalyst the Protector - more sustain, more hp, more mana.
Rod of Ages - we rush this item becouse we want to max the passive til the 26-26 min mark. If you farm good you won`t have problems to get the item in 17 mins.
Mercury's Treads - I hate to get cced while playing Cho`Gath but tbh anyboots are good for him. For mid lane tho the only other viable choice is 
Sorcerer's Shoes and i get them only if I`m 2-3 kills ahead of the mid. Then go on with the bulid listed above and prioritise 
Abyssal Scepter (that`s why you actually don`t need the [Sorcerer's Shoes ) If you face heavy ad team or just the enemy ad carry is fed after lane phase consider 
Frozen Heart before anything else.
