Most gyms prefer their instructors to be certified, and the certification program and exam are really good at making sure that you know what you're doing. When you go into the gym to talk to someone about instructing there, be confident in what you know. It's also helpful to have an idea of what you want to do for your class (what type of class you want to teach, what days and times you will be available, what equipment you will need, etc.) so that the owners can decide whether they need you or not. I worked at Snap Fitness in Cedar City, and the owners are super nice and friendly. It was such a great experience working for them.