There are many reasons why Cho'Gath is a good jungler:

#1: His passive grants a small amount of health and mana each time you kill a minion.

#2: His Q and W affects an area, making it really easy to rush the various jungle's monsters.

#3: His E, with a good positionning, also acts as an area effect. This means that Cho'Gath has 3 area skills that makes the jungling an easy task.

#4 Has a jungler, which according to me is less threatened by death, Cho'Gath can easily get his 6 stacks.

#5: His Q has a really good range, making ganks easier and more effective.

In this guide, runes are chosen for a maximum early game power. This will make it easier to get kills and get tanky quickly. The only possible change you could make for runes would be to get even more tanky but personnally, I think AP runes and magic penetration is the best

A mix of tank and AP masteries will be the best for Cho'Gath, granting him damage and renforcing the natural tank that this creature is.

In this list, I will give you the reasons of each items and some change that can be made according to the actual game you are playing in:

#1: Boots and hp pots: It is crucial that you get your boots. This make your ganks more effective. You will place your Q with ease and your powerful auto-attacks boosted by your E will happen much more often when following an enemy champion.

#2: Completing your boots with sorcerer's shoes: This is also crucial for its magic penetration and the speed it gives you. Rush this item as soon as possible.

#3: Rod of Ages: Absolutely the perfect item for a Cho'Gath jungler. It will give you a lot of health, mana, AP and this will upgrade over time (each minute to a maximum of 10 minutes).

#4: The Rabadon Deathcap: The item that will give you true AP. This is why I love Cho'Gath, he is a incredibly good tank but when built smartly, he also becomes a big part of the damages in team fights. This item will give you that damage.

#5: Rylai's Crystal Scepter: The item of choice to get more health and some AP. Definitively a good item for Cho'Gath. It will probably make him the most tanky in the game with its +500 health.

#6: This is the variable part. According to the enemy team and who is fed in it, you will either purchase a Thornmail (AD) or a Force of nature (AP). Finishing your build with the Abysal Scepter will give you more magic resist and remove some magic resist to surrounding enemys.

#1: Maxing your E: This is very important if you wanna go through your jungle with no pain.

#2: Maxing your W: Cho'Gath has not the longest silence ever, why would you not use it? This will make a huge difference in team fights and will also oblige the ganked to fall back immediatly.

#3: Your ult must be boosted as soon as possible, like most champs.

#4: Only set 1 point in your Q for the crowd control (CC) it gives. You will finish it at the end of your levels.

Smite: No need to explain this, it is a must have if you wanna jungle properly.

Flash: It as a really good escape that can pull you out of deadly situations. It can also be good for grabbing a kill.