I normally play AD Carries, and Kog'Maw is the Jax of AD Carries. Hitting with absurd attack speed and damage, as well as huge range, no champion can stop a late game Kog. The only problem is really your early game, which you're quite fragile and you need to play conservatively and try to avoid all possible harassment from the other team.
Pros / Cons
+Damage out the Wazoo
+Able to shoot for days with that range
+Even if focused, he can be a tremendous help in a team fight with his innate
+Good at chasing with his slow and ultimate
-Relatively weak early game if not fed
-Prone to harass
-Poor escape tactics from bad situations
First, you want to start off with your Bio-Arcane Barrage, allowing you to keep up harass on enemy champs bottom lane, while suffering little repercussions. Keep in mind that no enemy champion is safe from you when you have this active. Dealing percent damage is key to Kog'Maw, as late game, with his Bio-Arcane Barrage and a Madred's Bloodrazor, he deals out 10% flat damage against enemy tanks, in addition to his 341 normal AD. Max this skill first by level 9 for maximum range and damage.
The second skill you want to max is Caustic Spittle, which gives you a passive attack speed bonus, as well as an active armor reduction to enemy champions. When your friendly neighborhood jungler is in the river bush, be sure to hit the Caustic Spittle for maximum effectiveness.
Void Ooze is mainly just a utility spell in order to impede enemy escape from your lane, or to aide your own escape. Use it sparingly in order to conserve mana for your other spells, like Bio-Arcane Barrage.
Finally is your Ultimate, Living Artillery. Possibly the most useful ult in the game. Up every 2-3 seconds, you can fire it off as harass, but be careful, as the mana cost ups every consecutive time you use it within a given window. To avoid the extra mana cost, be sure to wait a few seconds between firing off your Living Artillery. The second part of this that makes it so versatile is that it gives vision on enemy champions when it hits, and also has a monster range. Be sure to utilize this aspect when chasing enemies through the jungle, as it can either help you clean up kills as a finishing move, or help keep vision as they attempt to flee through the jungle, where your team-mates can help pick up the kills.
As with any AD Carry, you normally want to start off with a Doran's Blade for the extra damage, health, and vamp. Initially, you could start with Boots of Speed and 3 Health Potions if you feel your early game is strong enough to go without the extra damage, health, and vamp from the Doran's Blade.
The first thing you want to build, right after your Doran's Blade, is Berserker's Greaves. Giving you the extra movement speed saves lives and you can increase the number of hits you get in while your Bio-Arcane Barrage is up.
Like I said earlier, your early game is fragile, but not completely helpless. After picking up your Grieves, you build Zeal, for the crits and attack speed, as well as to get your Phantom Dancer started.
By this point, you should be well off on attack speed, so you can now build your B. F. Sword. After this, you start to hit like a truck. At this stage, you should be starting to pick up kills to rack up enough gold to proceed onto the face rolling stage of this build.
Phantom Dancer is essential at this point. With your B. F. Sword giving you the necessary damage and your Bio-Arcane Barrage helping with that and boosting your range beyond absurd, more attack speed is better. If you get here early enough, then it's probably a surrender at 20 game for the other team.
With every Phantom Dancer, comes and Infinity Edge. Giving you the 250% critical hit damage, and increasing your damage and crit chance, even the tanks become afraid of you.
Next off, the The Bloodthirster gives you main sustain in team fights with the vamp and the extra damage is always welcome. The The Black Cleaver right after allows you to take out the tanky characters with ease with the armor reduction.
The shining crown jewel in this build though is your end game item. The Madred's Bloodrazor. With your Bio-Arcane Barrage active, you hit for 10% flat damage every hit. Tanks are taken out with ease. Even squishy characters run in fear from being two hitted. Late game Kog'Maw ends up being very capable to demolishing an entire team if not taken care of immediately. Although, another option is to just rush the Madred's Bloodrazor so that you can deal decent Attack Damage with good Attack Speed, while still getting you're 10% per hit once you hit level 9 and max your Bio-Arcane Barrage.