Kog'maw is one of the best champions in the game (at least thats my opinion) people may think hes squishy but for some reason i never feel that. maybe its because of his long range?
well anyway, in this guide i will tell you about how i play kog'maw as a main.

When the prophet Malzahar was reborn in Icathia, he was led there by an ominous voice which thereafter anchored itself to his psyche. From within, this voice bestowed upon him terrible purpose, and though Malzahar was no longer tormented by its call, the voice did not cease its unrelenting summons. This baleful beacon's gentle flicker -- now fastened to Runeterra -- drew forth a putrid beast that ambled across a threshold it did not understand, widening a fissure between the spaces which were never meant to meet. There amongst the haunting ruins of Icathia, Kog'Maw manifested in Valoran with unsettling curiosity. The spark which led him to Runeterra teased him still, urging him gently towards Malzahar. It also encouraged him to familiarize himself with his new environment, to the stark horror of everything he encountered on his journey.

The enchanting colors and aromas of Runeterra intoxicated Kog'Maw, and he explored the fruits of the strange world the only way he knew how: by devouring them. At first he sampled only the wild flora and fauna he happened across. As he traversed the parched Tempest Flats, however, he came upon a tribe of nomads. Seemingly unhampered by conventional rules of physics, Kog'Maw consumed every nomad and any obstacles they put in his way, amounting to many times his own mass and volume. The most composed of his victims may have had time to wonder if this was due to the caustic enzymes which stung the ground as they dripped from his gaping mouth, although such musings were abruptly concluded. When his wake of catastrophe reached the Institute of War, and enthusiastic Malzahar greeted him with an enticing prospect: taste the best Runeterra could offer...on the Fields of Justice.

'If that's just hungry, I don't want to see angry.' --Tryndamere, the Barbarian King

Pros: Cons:
long ranged attacks a little squishy
last hitting ultimate focused a lot
great slow
high attack speed

at first if your playing a 3v3 you should quickly buy your dorans blade and run into the brush. when you see an enemy coming you should shout your void ooze so that they get slowed greatly and your allies can charge onto them. after one of you gets the first blood you should quickly return.
if you want to win you have to get a tank on the same lane as you, then its gg. you may ask what causes this. well you should ask the tank to go and 2v1 them and right after you should activat your w and fire! then they will unknowingly be losing health, smart huh? well after this you will probably get a kill and kogmaw can even get fed with a first blood.

at mid game you will probably be only a little fed so you should focus on last hitting minions and champions to get even more fed. at the end of mid game you will probably have finished your Infinity edge. combined with madreds its devastating! just avoid feeding thats your second biggest goal

at late game all u have to do is right click and stand back and enjoy the gggodlikes your getting (ONLY if u havent fed)

at first you should get dorans blade and then continue the sequence until you have no more space then sell your dorans for a bloodthirster or black cleaver.

Occasional Items:

-Frozen mallet: if your having problems staying alive or chasing an enemy.
-quicksilver slash: if theres a ap burst mage on the other team
-hexdrinker: karthus
-thornmail: against heavy carries like tryn.