ADC is all about farming. Consistent CS is more important than getting early kills or even harassing your opponent. Practice last hitting and and always get the cannon minion.
Check scoreboard every so often to keep track of your CS compared to the enemy ADC. If they are CS'ing more than you or pretty close, then turn up the heat on them and ask your support to get more aggressive with zoning, you want to have a significant CS advantage over them.
Don't try to force first blood, if you do there is a 60% chance you will end up giving the ADC a double kill, and by doing that you just made the rest of early game much harder on you. Wait for your enemy to make a stupid dive, let the first blood come to you.
As much as I love kog'maw, he has been nerfed hard and struggles today. Ashe is in a similar predicament, in that her kit is outdated compared to today's high damage ADC's, but her CC can be used to great effect for her team.
are all in a pretty good spot right now.
is rather difficult to play with his axe mechanic, and his normal damage is somewhat mediocre, but if he gets early kills he snowballs like an absolute motherf*cker, god help you if he ever gets fed.
are assassin ADC's that don't play quite like regular ADC's in that you have to use their stealth and burst mechanics to really take advantage of their power.