For starters, practice farming. That cannot be stressed enough.
Go into a Custom match, pick the ADC you want to practice with, change your Runes so that you don't have a single bit of extra AD, and don't buy items. Just practice last hitting.
Then, when you go into an actual match, you'll be farming with little to no effort because you'll actually be building items/have Runes in those matches.
I would also use custom matches and pick a bunch of aggressive/annoying bots to play against and make it so that you'll be in Bot lane 2v1. You have to make your self-preservation become second nature, and that's one of the easier ways to do it since you won't be messing up a real person's game experience.
Naturally, this all won't teach you everything, but it made me a much better ADC because it taught me how to stay alive in lane.
As far as champions go, I would start with 
, 
, and maybe 
. There are good amounts of safety in all three of their kits, and they aren't hard to play.
is a good one, and isn't hard to learn, but she has a super weak lane phase. I would wait until you're at least a little more comfortable and have learned some of the matchups.
I would also learn everything you can about positioning.
Positioning is essentially the main thing to learn about ADC. It can make or break a fight.
I also HIGHLY recommend checking out Skill Capped if you haven't already.
Their videos are incredibly informative, and will help you to learn and understand so much more about the game.
Here's a link to one of their videos about laning as an ADC: https://www.youtube.com/watch?v=aHBZWKvih7o
And, one on how to avoid getting bodied when you're playing a stick in the mud ADC: https://www.youtube.com/watch?v=cqlJN3Pim5g
They have a vast amount more content than just those two, and I hope you check them out and get something from it.
Good luck on your quest to become an ADC!
I look forward to seeing you on The Rift! :D
.
.
