Hello! This is my first guide, so try not to be angry if you cant understand it. Kog'maw is my absolutely favorite adc. He can also play mid, but that is a COMPLETELY different story.

Pro:

Most range in the game
Great for team fights
Unlimited ultimate (which reveals!)
All-around amazing

Con:

Squishy
No escape
If not careful, very mana hungry

Passive: when you die, you run around for a few seconds, and then explode, dealing true damage to a small radius

Q: Deals magic damage, lowers their armor/magic resist for a few seconds, and gives passive attack speed. Has a medium radius

W: Gives you range, and deals a percentage of the opponents health as magic damage

E: Slows, and deals magic damage. Has a medium radius

R: Reveals, and stacks off ad and ap, deals %125 extra damage to champions, and takes extra mana every consecutive shot in every 6 seconds. Huge range

Marks: greater mark of attack damage
Seal: greater seal of armor
Glyph: greater glyph of scaling magic resist
Quintessence: greater quintessence of life steal

Traditional 24,6,0

Fury: 4, Brute force: 3, Martial mastery: 1, Warlord: 3, Devastating strikes: 3, Havoc: 1, Butcher: 1, Feast: 1, Executioner: 3, Dangerous game: 1, Spell weaving: 1, Blade weaving: 1, Frenzy: 1, Block: 2, Recovery: 2, Unyielding: 1, Veteran scars: 12.

start off with long sword and two health pots and a warding totem.
then gets berserker's greaves and a relic shield.
go for blade of the ruined king, runnan's hurricane, infinity edge, the bloodthirster, phantom dancer, and then sell your boots (with homeguard) for a trinity force.
(infinity edge or runnan's hurricane can be second, depending on if you are winning or losing)

q,w,q,e,q,r,q,w,q,w,r,w,w,e,r,e,e,e

If you are blue side, then taking golems for extra money, health, or xp, is not a bad idea. Also, as soon as you get your blade of the ruined king, you can solo dragon, although it is better to have someone with you. Whenever you can, try for the enemy's blue.

Your point in team fights is to melt the enemy champions. Go in with your slow, turn your extra range on, and pick them off with your artillery. You will be amazed at how fast their health goes down, tank or no tank! If you happen to die, try to use your passive to damage their team, or kill a runner (it deals TRUE damage). Remember: you are squishy, so stay far away from the center of the battle.

The most important thing to remember about kogmaw is that he has NO escape. So, farming is especially important for him. Don't push too hard, since ganks are easy kills for the enemy team. Also, don't be afraid to turn on your W for the extra range, so you cant be poked. Just farm, farm, farm!