You should wear slacks, a nice collared shirt with long sleeves. Some might say to wear a tie, but I say that is optional, depending on how comfortable you feel about it. Do not wear shorts or jeans.
Google GNC and read about the company so that you are knowledgeable about the firm.
If you are nervous, try breathing deeply a few times before the interview and think positively about getting the job. Picture yourself actually getting the job offer.
Employers are looking for someone who is trustworthy, on time, reliable, good with customers, works well with co-workers, is willing to learn, and who had a good attitude. Enthusiasm goes a long way, if you can convey interest in the company and in working there.
It is always nerve wracking to interview and get a first job, but stay with your own internal thoughts and what you want to present. You are selling yourself to the interviewer so be candid about your strengths and desire to perform well for the company on the job.