If someone wants to start learning ADC, which champion is best to start with?

Ehhh, she's always a good fallback, but some better ones might be Tristana (safer, has her own self-peel) or Caitlyn (another safe laner, but with a different style then Trist, more reliant on early bullying and then zone control sieging mid to late game)

Either of those are good picks to start with, plus you can get Trist for free (with her Riot Girl skin!) by following Riot on their social media platforms. I forget which exactly she is, either YouTube, Facebook, or Twitter

To learn ADC - you want good fundamentals on positioning. I think a lot of the ADC champions will teach that poor positioning is okay, like Ezreal and Tristana. (What?! you caught me? no problem! i got an ESCAPE!)

Playing an ADC with NO escape FORCES you to understand how critically important your position is. For that singular reason, I'd highly reccomend starting as Ashe.. You will die more often, and probably lose more, but you will LEARN more as well.

My second choice would actually be

You also realised that the meta is shit now, and the only way to climb is to hop on the Adc meta train huh?

is easier and safer

Caitlyn, tristana

probably. That way while learning finer positioning you'll still contribute a decent chunk of damage.

The problem I have with Caitlyn is that people will get too used too with the range of her AA so if they play other ADC's they will get too close.

ashe just has insane kiting and her kit is simple. my problem is how I can do fine with tristana, twitch, even kalista, but for some reason I'm complete shit with ashe, and I cant explain why.

but id recommend or cause once you hit that 30 minuet mark it dosent matter how hard they kept you down, because people don't know how to close out games this far down the ladder

Ashe and Miss Fortune are probably the two more forgiving of the ADCs, so I'd recommend them. Personally I prefer MF more, but Ashe is generally easy to play as well.

I think a good starter ADC to learn is Sivir. She has wave clear so you'll never be pushed in and forced to farm under tower, and she has 2 get out of jail abilities with her spell shield and ult, if you mess up. That said I feel like mobility is so important it might be a good idea to pick up someone like Trist.