Do not wear jeans. I dont care if you're gonna be working at Spencers, its not professional. Closed toe shoes. No sneakers, or flip-flops. If you are interviewing at a clothing store, its always good to wear a piece of their clothing. Just make sure you look professional.
Speak clearly, and think about questions before you answer them. (I'd rather someone take the time to gather their thoughts, than stumble over themselves trying to figure out what they are trying to say while speaking). If you're nervous, say you're nervous, people are sympathetic.
At the end of the interview, definately thank the interviewer for their time and considering you for the position. Do not say "please hire me" or anything like that, sounds desperate. Just as effective is "i'm really looking forward to hearing back from you"
Now - this being your first interview, i'm assuming its your first job. Talk that up! Say that you're ready to join the work force, and how you think that particular company would fit your personality. You are open for training and suggestions that will help set a foundation for your future (They will ask you what your future plans are - its totally ok to say you havent decided yet) Most people are reluctant to hire people with no experience, but if you play yourself up someone willing to learn and easily molded... you have a good shot
Former Retail store manager... conducted countless interviews.