Well, I'm only 14 so here's my advice...
I do believe that there's a difference between sales associate and cashier. A sales associate would walk around the store making sure everybody is pleased, and finding what they're looking for -- you know, those types of people you find in stores. A cashier has to stay at her assigned till and sell items -- I'm sure you know what a cashier does. Hahah.
In my opinion, being a sales associate would be way fun! (I just hate math and could never be a cashier) It also boosts up your social skills. :)
As for your resume, I think you should totally mention your past cashier experience! If you have any rewarding grades or honourable mentions at school, or through other programs, jot those down as well! Your resume should show your personal accomplishments and the tasks you can handle. For example, you have dealt with a diverse customer population, handled customer inquiries and solved billing problems, you are enthusiastic and cooperative, have received %90 averages in math, very social and loves to help people, etc.
Just put things about you! You said you have experience as a cashier, so take a look at your past resume if they retrieved it back to you, and see what elements you added on there that may have got you hired. Make sure it is formally typed as well. Nothing like "hEy ThIs Is My ReSuMeeee? Hyy-re MeH!"
Oh and another smart move would be, don't put anything dumb like "how much does the job pay?", and definatly don't bring that matter up on your interview... let the boss tell you. Also at your interview, act cool and bussiness-like, to show that you have a firm head on your shoulders and know what you are doing!
Dress nice too!
The store you choose to apply for is totally up to you! I find H&M is an atmosphere where people just come and go as they please, though. And if you can take the stronge aromas from Abercrombie, then that could be an option too. A clothing store would be your best bet over all! I really want to say Hollister is a great place to work at, but that is also your choice. First check if the stores are even allowing new employees. Then travel to each one, and see which one you think you would belong into most. It's sorta a personal thing for that matter. But whatever you choose, have fun with the discounts, and your fellow employees. Oh, and good luck! :)
I'm pretty sure some stores would accept you.
Two friends of mine work at the Royal Ashburn golf course down in Whitby, or wherever it is. (haha, I forget. :P) They got hired right away as waitresses, and they're only fifteen as well. So I'm sure you can get in! :)
Holiday seasons are always busy and looking for the most help they can get, so you will definatly be able to work during that time!
Hope I helped! :)