Firstly, you will need to make sure what you say is original, and that you are passionate about the role.
If you can see yourself working your way up the retail ladder in later years, then make sure that you say this. Tell your interviewer that you are so eager to get the role because you can see yourself as a designer, or that you dream of being in a managerial role.
You also need to get across that you are interested particularly in the role that this interview is for. For example, why you want to have a career in fashion. Is it because it helps you express yourself? You feel that you will be able to be creative? You love putting new outfits together?
With any retail based job, you will be working with members of the public. So you will need to make sure you tell your interviewer that you are a people person, and that you enjoy helping people on a day to day basis.
With all interview questions, it is important to make sure your answer is personal, so tailor your answer to suit your aspirations and just be passionate! A potential employer is much more likely to hire someone if they believe that they really do want the job.