since it sounds like a retail job, dress cleanly and have an upbeat/positive attitude, even if you are nervous. you may not be excited for the job but at least look like you really want it and that you're eager to do well. that'll probably be more important than any nervousness you exude while at the interview.
try to calm down by controlling your breathing and dont worry about needing a moment to think to give a good answer. having good responses where you dont have to backpedal or ramble helps you break out of being nervous and/or having a shaky voice