For what to wear, wear something smart so if your male shirt tie and smart trousers, if your female try a blouse/shirt with a single colour knee length skirt and smart shoes. The interview will include questions as to why you want to work there what you can bring to the job, they might ask how you would deal with a certain situation, what you know about the company (id reccomend getting some facts memorised about it just some small ones to show you know something about them). with regards to pay for under 18s it can be from £4.20 as that is the minimum wage. if you need anything else id be happy to help