Good question. I would say just about what you would wear to any interview. Although as it is for waitressing I would forgo the trousers and go with the skirt (knee length shortest!) with tights or stockings. White blouse, and jacket. Hair tied back if it is long (as it will need to be if you get the job), minimal makeup – dont go in heavy make up. Nice nails, manicure before you go if you can, so they all look nice and neat