Hello people.
This is my guide for kog'maw. First one too.

I hope you like it. Give feedback and try it first before you vote. If you vote down, please tell me what.

Alright you start with an doran's blade. Why maybe you ask? It's all good for an good start.
When you get 1000 gold you buy berserker greaves. Awesome attack speed. Then your first real item will be The awesome malady. Why? Have you ever wondered about your W? Malady reduces their magic resist which equals that if they have bought none magic resist items, then you will do almost 6% damage on them EVERY HIT! Imagine that? 6 x 10 = 60%! 10 hits and they are already running to get back to base. But i'm not even counting your base attack in this calculation. And why max E and not any other spell? Because E gives you an great damage output in early game. It's easy to hit with, and the slow is awfully great for your Ulti and everything else.

I take FLAT ATK Speed runes. For malady to gain maximum damage in the beginning. You can also try with HP runes.

Kog'Maw in my opinion is a HP eating beast. Shred their magic resist and eat their HP with W.
Sometimes you can even spit a Q just for good measure, that reduces their magic resist to ZERO. Eat em up, and eat em up good. :)