OK so I main Graves and Corki...
Start with the easiest ADC to learn, Ashe Cait. Abuse her range, poke enemy laners down.
First things first, focus on your CS-ing. Get into a custom game and just AFK farm. Get at least 110 CS @ 15 minutes. Get used to each ADC's auto attack animation and travel speed and sound and everything. As an ADC your priority is to farm unless you're ridiculously ahead (WITH ITEMS) then you can go ahead and group with your team.
Second, trading. As Caitlyn this is really easy to abuse. You can farm safely from a distance with 650 range and Q, and bully enemy laners out since they can't auto you. Also, you should only trade with enemy laner if you know you can win the trade. Or else, back off.
EXAMPLE: Cait > Vayne pre-6, so as Caitlyn you should keep harassing her, and she'll cry. Try to shut her down if possible, like asking ganks.
Vayne > Cait post-6, unless you're really ahead as Caitlyn. Respect Vayne (more) if she already has Bilgewater Cutlass. At this point of the game as Caitlyn if Vayne knows that she's just going to keep getting stronger, you need to end the game ASAP. Vayne will outscale you so hard late game it's unreal. Unless you're really ahead.
Third, orbwalk. After you cast your auto animation and it did damage, you should click a random place to cancel the whole auto animation projectile. Makes you dodge skillshots more easier and makes you look like a professional ADC main. Kappa
Fourth, kiting. TBH I think almost all of the ADCs are absolutely great kiters, especially Vayne, Ashe. Kiting with Graves, Lucian for me, requires almost no skill if you main ADC. Kiting with Caitlyn, Vayne, Ashe, kinda requires skill. I'll just leave it here, I have no place to talk since I'm bad at kiting too...