First off, Kog and Twitch arent easy ADC's. Twitch a bit more, but Kog, you better have your positioning down pat.
Now my ADVICE. First off, a good deal of learning ADC, is learning all the ADC's and all the SUPPORTS to some degree. Knowing what skills an enemy ADC may use, and an enemy SUPPORT may use, are vital to you knowing how to stay alive, when to be aggressive, when to be sae. Champ ranges and damages. Part of that you can learn just by playing against them, but also as dude said, try them when they are free. Will help you even once you get a preferred adc.
TIPS - Communication with your Support is vital. You guys MUST be on the same page. Let them know if your going to be aggressive, let them know if you want to hold back and farm. ect.
-BE SURE YOUR WARDED. Especially if your pushing. Dont just assume Support is buying wards. If they are great, but if you need to as well, do it. If they onlt throw one in the bush, make sure there is another in Tribush if your top, or farther up river if your bottom. The extra time knoowing when a gank is incoming is invaluable.
-BE CAREFUL EXNTENDING LANE. If your going to have a pushed lane, make sure you keep your mobility/escape skill ready. Dont OOM. Be sure your warded, this time above all. A gank when your pushed to far can really set you back hard.
-KEEP MOVING. Alot if not all ADC's have some poke skill. Dont get hit by it. For that matter dont get hit by the supports either. Keep moving, so you cant easily be poked and dove on or forced B cause your low. Dont stand in one spot and hit creeps.
-LAST HITTING. Ultra important, of course. ALSO, dont just auto hit every minion, if you werent aware of this, though I assume you are. If you dont need to push lane, dont. Freeze it and just farm gold.
-DONT BE NEEDLESSLY AGGRESSIVE. 15 minions is a kill esssentially. Dont waste skills and resources poking and attacking enemies just to run out with no kills and a bunch of lost last hits. You will end up behind.
All I can think of atm, I've had a few beers.