Never worked for AF but I did work for one of their competitors for about 6 months. Basically they will either hire you as, what they call a "model," the people that actually work on the floor and sell clothing or as support staff (you'll work in the back or in the shop when there are no customers), they have some name for them too but I can't remember it at the moment. If you want to be a "model" you better be either a fairly ripped guy or a skinny attractive girl. The interview shouldn't be terribly hard, as said previously it may be a group interivew, they will most likely ask you the generic interview questions you can find any where on the web and some questions more geared towards their brand. Just go in looking your best, wearing a dress shirt and slacks, even at an AF interview it could up your chances of getting the job, when I went for applications and I went to American Eagle I got hired right away soley because I dressed up.
On another note, have you ever worked retail before? I personally would advise you against it if you have any other options (other than fast food). It really does suck as much as everyone says. You will work late hours, sometimes you will be required to work overnight, likely they will get away with not paying you overtime (At AE I worked from 7pm to 6am without overtime because the shift was split over 2 days), and you will proably do this close to once a month. The pay stinks, minimum wage usually, opportunities for advancement are few and far between, and you most likely will spend alot of your money on AF clothing because it will be "required" for work (they can't tell you that you have to wear their clothing to work but in reallity you do). I really suggest that you try to get an office job, data entry, or even work as a host/hostess at a local restaurant, I personally have had far better experiences with local small business than I have with big companies.