Your ultimate is a really good way to poke and keep the enemy AD Carries HP lower than your own.

Always pop the ultimate a little bit behind the AD Carry, because most of the times the AD Carry will move in the opposite direction from you to dodge your ultimate, and this will cause him to run into your ultimate. If the enemy AD Carry starts to dodge your ultimate by running to the sides, try to learn in with direction he runs so you can try to pop the ultimate on that side of the AD Carry, and also you can try to use your slow to have a bigger chance of getting your ultimate right.

NOTE

Always wait 6 seconds untill you pop the ultimate after the last, if you do that the mana cost of the ultimate will not increase and you will not get out of mana too early.

Your "W" is your main source of damage.

Always pop your "W" when you start attacking their AD Carry.
Your "W" also gives you attack range, so you can use it to poke from a distance where their AD Carry can't reach you. But remember that if their AD Carry is, for example Ezreal, he can get in range almost instantly, so I don't recommend pokeing champions like him with your "W", wait untill you are level 6 and then poke with your ultimate instead.

Kog'Maw is not good at mobility, so you need a support with good crowd control like:
Alistar or Leona to keep their AD Carry Stunned.

Kog'Maw is a really slow champ with no mobility abilities, so make sure your support keeps the lane warded, and if you get ganked, make sure you start running towards your tower in time.