As an intern, you will be doing mostly grunt work and running errands. So they won't expect you to have a lot of "fashion" background yet. It will pretty much be like any job interview, so I'd put together a resume, if you havn't already ... even if you have not worked before. There's still achievements in school and clubs that can show that you are a good candidate.
But you can impress them with showing up neatly groomed, being on time and by just being polite and friendly in the interview.
They will probably ask about any realted experience ... so you can tell them about any jobs you have had ... realted or not ... and scholastic backgroundand and classses, any organizations that you were involved with, any offices that you may have held and any other acheivements that have prepared you to work in business, retail or fashion (stuff that you should already have on your resume).
You want to impress them that you are dedicated to doing the work and the you are reliable. Wanting to learn is important, but that is the reward for doing the work ... you are there to work, first and foremost ... so let them know that you understand that.
They will probably ask why you want to intern with them, so I'd have a logical answer for that, maybe stating how that will assist you with your long term goals (having short and long term goals is a good thing and show that you are serious).
I'd do some homework on the company as well, ... know things like who and what their market is, where they are headquartered, how they got started, and read any recent press releases to find out anything really important that may be impacting the company and it's employees like expansion or adding divisions. That kind of information will be on their corporate web site ... look for "investor relations" and "press" links. If you can ask them an intelligent question that shows that you have a bit of knowlege about what they do or where they are going, it would really impress the interviewer.
Don't ask questions about how they can help you ... like pay (probably none) or taking days off ... it will only make you look like you are not dedicated. Most of that information will come up ... if not in the interview, then later if they offer you the internship ... and that would be the best time to ask, if they haven't given you that information yet.