I am a gold 1 adc main
Kog'Maw has been one of my favorite champions since back to season 2 when he was really strong.I main ADC and back in season 3 I had my most ranked games played on Kog. Recently Kog'Maw has gotten a buff that allows his W Bio-Arcane Barrageto cost no mana and his Q Caustic Spittle has become a skill shot. Kog's Q now has an increased range, does a bit more damage and shreds a lot more armor. This means he is even stronger in the late game because you don't have to get close to shred armor. The no manna cost on his W makes him easier to lane because you don't run out as quickly as before.
This guide is in depth and says things that are very basic about ADC but is important for players new to ADC

Laning can be a bit weird with Kog'Maw if you are not use to him because of how Jurassic ( Jurassic Kog'Maw pun) his auto attack range varies. Kog does most of his damage in the late game so the longer you drag the game on the stronger he gets. Now just like every ADC if you get fed in the laning phase of course you will do really well in team fights but just like Vayne, Kog does more damage the more health the enemy has, that is why Kog is a great pick into enemy teams that have very healthy tanks such as Mundo. While you're laning harass is an important part of playing Kog because his range can catch people by surprise. The key is to turn your Bio-Arcane Barrage on and auto attack them while they are getting cs so it is hard for them to trade. Also while Kog has his W active he has one of the largest auto attack ranges in the game. If you are going for more harass use your E Void Ooze which slows them to get a few more autos in.
It is very important to use your Caustic Spittle properly now that it is useful it can help you do more damage. This ability is also an auto reset to use after an auto to get an extra in. When you and your support go all in on their bot lane hit the ADC with your q and then auto with your W on, it will significantly increase your damage output. Your E is very useful if they decide to run because if maxed second it has a large slow.
Kog's ult Living Artillery is one of the best harass in the game because it can constantly be used with a low cool-down and low manna cost if used properly. When you see a friendly minion on low health and their ADC walk up to get the CS put your ult where they have to walk. They either get the CS and take damage or miss the CS. The only time you want to use your ult multiple times in a row is when you are fighting or you can grab the kill when they are escaping. Kog's ult combined with auto attack does some of the highest trade damage in the game due to the proc'ing of trinity force.
Landing a good E in lane can lead to 1-2 guaranteed ults to follow Don't be afraid to buy health and manna pots because they are important but do not rely on them maybe 2 Mana Potions at most but you shouldn't be spamming ults to wasting mana.

I almost always rush trinity force do to its power and trading potential. The key is to an effective trade is to spam the ult to proc the trinity force passive while autoing. Kogs ult cannot be spammed because its cool-down is shorter than the cd of trinity so you will have to wait and get use to the cd of trinity. Trinity is ver expensive and is your first power spike in the game usually if the lane has been going even then the trinity will make you win trades.

long swrd + 3x Health Potions is the start I go if I know it will be a hard lane such as against a Draven or have a support with no heals/sheilds. The long sword can be built into a vamp septar to help with substaining in lane till the late game.

As Kog you're going to want to do some good early damage. I usually start with Doran's Blade and 1 Health Potion. Doran's Blade returns life (HP) on hit, allowing trades. and the health pot is cause you have extra gold and is nice if you get harassed.

Trinity Force is a great because of it extra damage and all around stats. Blade of the Ruined King has a very nice passive that works well with Kog's W. The active can make Kog great for 1v1'ing, kiting, and chunking tanks.

If you are ahead I would rush Trinity Force then Infinity Edge. This because these items are great for team fighting. If you are able to position well you can easily carry

Kog's ult has a far range that makes it easy to harass and give vision of the enemy before team fights. You should use your ult to face check brushes from a distance so you don't get amBUSHED. No but seriously it is very helpful. Your role in team fights is to sit in the far back and auto attack their team. Most of the time it is okay to focus the tank because that's who you do the most damage to. That would be a perfect game for you, just to sit in the back and auto while ulting but that almost never happens. There are champions like Jax, Irelia and tanks that want to focus you. It is important to know how the enemy champion will engage on you, Jax and Irelia have a gap closer while a champion like Darius will have to walk at you. This is where you have to know how to use your E, if the enemy is coming to focus you and has a no gap closer use your E at them so they have to walk all the way through it to get to you. If the champion has a gap closer and can jump to you use your E behind you to kite walk them and make them chase you through it. Also once then get close to you use your Blade of the Ruined king because it will speed yourself up and help you escape. ask your team for at least one person to peel if you keep dying in the beginning of the fight. Clean up is easy with your ult.