It's a typical sales interview - be prepared to sell yourself with a smile and confidence, that is all that's required. Maintain a positive attitude and be extremely friendly, that's what they are looking for. I would recommend practising looking sharp and rehearse all of the reasons why you're going to succeed in the business. Let them know that you are a hard worker, willing to dedicate your time to the job, are good at making friends, and always keep a positive attitude. They will teach how you to do sales (the exact things to say and do to make people more likely to buy) but it helps if you are naturally a friendly and likable person. Show up to the interview wearing nice, crisp slacks and a good ironed button down shirt and tie.