I would always dress smartly – this doesnt always mean a suit by the way!
Its likely to be the standard interview questions and probably minimum wage which is fine for someone just starting out or needing any job to bring in money. Try to familiarise yourself with the range of products they sell (and i mean men/woman/child approx sizes etc) to show some knowledge as the more knowledge you have about the products/company the less training you will need if successful