Tristana, Ashe, and Sivir are good for first timer ADC. (I don't know alot about corki)
Sivir is very good pick against a blitzcrank as each blitzcrank pull when you activate spell shield, will negate the damage and the pull AND give you more mana!
Ashe is a good global ult learner. Sh'll help you get used to others like Ezreal and Draven's ults. She has a perma slow so even if you dont get alot of damage out you can help your team by NOT feeding and then slowing enemies for your team to kill them.
Tristana is perhaps one of the safest of the 3 ADCs Haveing a rocketjump, a ult that blows people away, AND flash summoner. You have quite a few escapes for when you find yourself in a bad situation!
On all of theses I find it best to build
makes your crits do 250% instead of 200% All of the champions above scale great off crits and attack speed..
< - A good example of a bronzie item build path for an ADC. exactly in that order..
but they say its best to build accordingly..
NEVER and I repeat NEVER build
or
AS A FIRST ITEM.. build DAMAGE first.. I never do good building this stuff Frst.. not even if Im playing
I ALWAYS need damage first...
THis may not be the most UP - Beat videos... but they're pretty legit.. just slightly listen to this and you'll perhaps learn quite a bit for ADCing.
https://www.youtube.com/watch?v=1diu8neEGhg - Laning
https://www.youtube.com/watch?v=NmsugUUANPc - positioning.
Dont Try to force yourself into a role.
This is my last tip, not every role is always right for everyone.. Otherwise I'd be a professional Filler.. There are people out there who can fill any role, but many people can't do every role.. I have another friend who can do Mid Jungle and support.. but they can't do ADC... And then I have a friend who can do anything... (Lucky guy....)
You may have a hard time with a role... You have a hard tie with ADC... I have a hard time in the jungle... I tried soo many times to jungle.. No matter what, I can't seem to get better as it..
so If I go into champion select I inform people..
"Anywhere but jungle" That's not too much for me to ask I'm at least not one of those "MID OR FEED" people... There are people who can switch up their roles and work around my disability to jungle.. and almost every time there's something willing to take jungle for me.. and I show my gratitude by thanking them in chat AND usually in the form of a good leash...
#Edit
I found a Scrap computer guide for corki! he's really great at detailed guides and is usually up-beat to listen to.
https://www.youtube.com/watch?v=Bznb3CFzcew
See if you can learn something for corki. :)
http://prntscr.com/8e2rn1 OH LOOK! You learn how to corki well and people might hail you as a Hero among ADCs... YOU COULD COUNTER VAYNE!