How to be usefull as an ad carry?

So i always had a 'crush' on the ad carry role but i cant play it at all. I can farm decently well but have no idea how to trade or fight efficiently. If i win lane im probably playing varus or gettign carried by my support but i kinda dont like varus that much right now. I have played a couple games of kaisa and other adc's and even at 3 items i feel im useless due to my dmg. I am kinda decent on attack-move and sometimes i use T which is the hotkey i have put on the slot of hotkeys then go to player movement and then player attack move click but its kinda bad due to targeting the target closest to my cursor so i do even less dmg. At least i feel this way. I know that theres a setting where i can make my left click be an auto attack but i cant get used to it since i never really needed it im jungle main with mid secondary and T is working fine but on the ad carry role im lacking. Do you have any ways of helping me out or tell me adc's that are good for me to try right now my eyes are on lucian and vayne but i do know that they are really hard champions. Any suggestions tips or help is apreciated. Thanks!