This article is made by lol forum moderator zerofin. Here's a list of his personal opinion which ones are the best for people to start with, notice it's his personal opinion so by no means the best choices for everyone. Some people may find other champions better for beginners!
Stands for Attack damage carry. ADC is most of the time an range AD based champion that lanes at bottom with a support. Their job is to deal damage to whoever is closest and stay at a safe place. Yes, stay at a safe place and attack the closest target, even if it's a 300 armor Malphite. That is your job as AD. A lot of people might yell at you that you are focusing their tank instead of focusing their carry. Of course, you can focus their carry if it's safe to go for them, but if there's their tanks blocking the road and they are staying behind, it's not worth going for them. Because by the time you get there and shoot them few times, you'll most likely be dead already.
Just a general info to give you the mindset how you should be playing the AD, not going too deep with this as this thread's point isn't that anyways. lol
In my opinion one of the best AD carries to start with. Firstly, she is free (Liking Riot on Facebook) so if you would like to try how does beng ADC feels like it's a really good choice.
The fact I put her at top 1 is because of that her kit is really beginner friendly. You get a jump which can be used for escape from bad situations which might happen often because of the bad positioning as beginners can't do it that well yet. Also on top of that her passive gives you extra attack range, and when you are lvl 18 your attack range boost will be really helpful for positioning even if you aren't good at the beginning with doing that.
Last hitting in tower might be slightly harder for her due the explosion shot dealing damage AoE to the mobs so you won't be able to easily last hit "normally", which means melee minions would be 2 turret hits then you can last hit and range minions 1 turret hit and your last hit, but you'll get used to it real quick.
She's really cheap, few games and you'll have enough IP for her. She won't be one of the easiest AD to play, but she is really cheap and good for learning how to play AD the fact that positioning with Ashe is really important because she doesn't have any kind of speed buff, gap closer and a small attack range. Only thing she has is frost shot/volley which slows and ulti that stuns, so with this at the same time you'll learn how to kite as an AD. Just make sure you take an advantage of your frost shot on kiting. Hit the enemy, then back off abit and then hit them again. Don't stand still. It might be hard at the beginning, but once you get used to it, you'll perform a lot better with teamfights.
Honestly, I would rather skip the third AD spot because a cheap and easy to play ADC doesn't really exist apart from the 2 listed above lol. But easiest to play with low price would be Sivir I suppose. She won't be the easiest one to play in team fights, that's for sure, because of her low range and no CC/escape at all, so if they have any hard engages or assassins you'll be quite in a trouble, let's say if you are off position and they have a Fizz that notices you off position, you are in quite a trouble lol.
Apart from that, her laning is quite easy due the easy wave clean with the boomerang and ricochet. Unless if your enemy is something that has good poke or long range, that way they could easily deny you from the minions, Sivir being small range and stuff lol. She is best to be played offensively, in my opinion. Something like Alistar/Taric as support makes the lane really dangerous for the enemies because if both you and Taric get all the skills off, their AD/support is quite much dead.
(Notice : Both Ashe and Sivir are really easy to get, and fairly easy to learn how to play them, but to perform well with them, that's something harder.)