this is my first build so i hope it'll be usefull.
i created this build trying to keep magic resist and armor the same in combination with high HP and AP. i mean how many times haven't you had it? you're an AP deal greta damage but dun got enuf resistances of HP to tank. you've got many resistances and HP but deal almost no damage.togheter with that cho isn't the fastest champion of the game so you need to take them out quickly, else they'll escape quiet easy. in this guild i'm gonna make sure you can tank, deal massive damage and make sure ur enemies won't escape.

Greater Mark of Insight
+0.95 magic penetration

i choose this rune to make you able to use ur skills on full potential even if the enemies have magic resistance

Greater Seal of Resilience
+1.41 armor

and good item to keep ATD champions like warwick off, this iwll make sure that every basic attack of an enemy gets reduced might help alot when getting ganked or so ever

Greater Glyph of Focus
-0.65% cooldowns

the cooldowns of cho are quiet long and since you're a mage build on AP you'll need them alot that's why i chosen this rune it makes you able to launch more skills in shorter time and also you'll be able to stack up faster with Feast since that cooldown wil also get reduced

Greater Quintessence of Fortitude
+26 health

i token this rune because well...you're cho his main defence is his huge HP so i wanted to improve that even more

i focused the masteries on both offense and defense. i'd recomment to first do the defense masteries then the offense. offense might be great for dealing damage but if you first make sure defense is done it'll keep you longer in the battle. dealing great damage is ofcourse usefull but what's the use if u start with an kill attempt but can't finish it because u also almost die

Regrowth Pendant
15 Health Per 5 Sec.

this is easily and shortyly said to keep you longer in battle in begin being able to farm more minions and lvl up faster

Mercury's Treads
25 Magic Resist. UNIQUE Passive: Enhanced Movement 2 , 35 Tenacity

this is for both magic resistance and tenacy. you're a tank they'll most likely try to take you out first and make sure you won't get away with stuns, slows etc this will help you shorten them

Needlessly Large Rod
80 Ability Power.

this will help you get started with dealing damage. futher nothing special about it

Rylai's Crystal Scepter
80 Ability Power, 500 Health. Passive: Dealing spell damage slows the target's movement speed by 35% for 1.5 seconds (15% for multi-target spells).

this is an awsome item it'll give you +HP +AP and it'll slow your enemies with every magic attack and since even your normal attack is magic because of Vorpal Spikes so you'll slow enemies every single attack and this in combonation with Rupture will slow the enemy for a HUGE amount. it can both help you chase your enemies and help you escape.

Shurelya's Reverie
330 Health, 30 Health Per 5 Sec, 15 Mana Per 5 Sec. UNIQUE Passive: Reduces ability cooldowns by 15%. UNIQUE Active: Nearby champions gain 40% movement speed for 3 seconds (60 second cooldown).

this will help you out alot in the first place it gives HP and it makes you regenerate both mana and HP faster. also it's passive and active skills are awsome. you can use the active when ganking enemies and when you and your friends need to escape a gank

Rabadon's Deathcap
155 Ability Power. UNIQUE Passive: Increased ability power by 30%.

+155 AP +30% AP shortly said amazing item to deal massive damage. no futher ocmments

Guardian Angel
68 Armor, 38 Magic Resist. UNIQUE Passive: Revives your Champion upon death, restoring 750 Health and 350 Mana, this effect can only occur once every 5 minutes.

this item is great gives armor magic resistance and gives you an second chance to take vengeance on enemies after you died when they used all their skills, used their mana, are on low hp.

Abyssal Scepter
70 Ability Power, 57 Magic Resist. UNIQUE Aura: Reduces the Magic Resist of nearby enemy champions by 20.

gives ability power and it reduces magic resistance of nearby enemies so double effect i also chosen this to get magic resistance about the same amount as armor again

Carnivore
Whenever Cho'Gath kills a unit, he recovers 40 - 82 (based on level) health and 8-16 mana.

awsome passive ability not the best but sitll usefull since in combonation with Vorpal Spikes you can kill more then 1 minions or creep at the time and you'll be able to regenerate quiet fast

Rupture
Ruptures the ground at target location, popping enemy units into the air, dealing damage and slowing them.
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 (+1) magic damage, and have their movement speed slowed by 60% for 3 seconds.
Cost90/90/90/90/90 Mana Range 950

awsome skill since it'll slow the enemies make their chance of escape smaller it tosses them up the air what will also give you time and when fleeing you can use this to create distance between you and your enemy. it comes to it's max potential with Rylai's Crystal Scepter this will make your enemies almost unable to move. so i chosen to max this out first for both it's chasing possibilities and fleeing possibilities

Feral Scream

Cho'Gath unleashes a terrible scream at enemies in a cone, dealing magic damage and Silencing enemies for a few seconds.
Cho'Gath screams in a cone in front of him, silencing enemies for 2/2.25/2.5/2.75/3 second(s) and causing 75/125/175/225/275 (+0.7) magic damage.
Cost70/80/90/100/110 Mana Range 300

feral scream i mostly use it in combonation with Rupture this will fully disable the enemy but i chose to max it as last because i see more potential in the other skills

Vorpal Spikes
Cho'Gath's attacks passively release deadly spikes, dealing damage to all enemy units in front of him.
Toggle: Cho'Gath's physical attacks launch spikes that deal 20/35/50/65/80 (+0.3) magic damage to a line in front of him.
mana Cost No Cost Range 40

i chosen to max this out as second because of it's farming abilities... it makes you able to lvl up quicker and when your other skills are in cooldown you can still deal quiet much damage with these

Feast
Devours an enemy unit, dealing a high amount of damage that ignores magic resist. If the target is killed, Cho'Gath grows, gaining maximum health (maximum 6 stacks). Cho'Gath loses up to 3 stacks upon death.
Deals 300/475/650 (+0.7) true damage to target champion, or 1000 true 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 3 stacks upon death.
Cost 100/100/100 Mana Range 100

great ulti, it brings massive true damage and it ignores magic resistance also it will be great help in stacking up your HP. when you're havbing haqrd time feasting on enemies just feast on creeps or minions till you completed your stack then you can use this ulti as an great damager even if your enemy is with full HP since then it isn't neccesary to kill the enemy with it

Teleport
After 4 seconds, teleports your champion to target friendly non-champion unit. 3 minute cooldown if cancelled.

i chosen this spell coz being cho gath you can be a great carry but must always think of the fact that ur a tank, a living meat shield so when a friend is in need of help u should be there as quick as possible

Clarity
Restores 160 + 30 x level mana to your champion and 50% of that to nearby allies within 600 range

i chosen this spell because i hate to be out of mana mostly in begin of the game, you can do like 3 casts and then you're already out of mana even thought later in the game it will be kinda useless u wont need it alot anymore

alternative:
Exhaust
Reduces a target's Attack Damage by 70%, Ability/Item damage by 35% and slows its Movement Speed by 40% for 3 seconds. 550 Range (estimate).

usefull when chasing enemies but i think this is kinda useless because of Rylai's Crystal Scepter this will already slow the enemies

Flash
Teleports your Champion to your mouse cursor. 475 range.

quiet a good spell it can get you fast to your enemy and away from your enemy, I my self don't use it alot with cho because i always slow the enemies, but seen to the fact that cho isn't the fastest champion in this game it might be usefull too. and it's also usefull to escape ganks sometimes