This varies, but usually the manager will provide instructions and expectations. Typically you'll be asked to teach a warmup, some section in the middle of the class, and then a cool down. You want to show the manager that you are able to teach, keep the class energetic and engaged, and can effectively deliver a safe environment. Also, put yourself in a good mood! Nothing to be nervous about. In the meantime, just come up with a warmup and cooldown, and write down all exercises you're going to teach that day (include variations).