Come with a list (physical or mental) and description of all your previous experiences. That could include experience with gymnastics, coaching a sport, or even just working with children. Dress nicely but you don't have to be super fancy (don't wear heals if they are not comfortable), and be openminded. Being a coach at the Y is a lot different from being a coach at other gyms since you have to set up and set down the gym. Also, they WILL ask you about a time you demonstrated leadership so come with an answer already in mind.