Take many different fitness classes. The best way to learn to teach classes is to observe others teaching them. Take notes of what appeals to you as a student so you can later incorporate those things into your instruction style. You can also note what you dislike.
Read articles from credible publications and organizations that cover fitness-related topics. These may include such topics as exercise psychology, basic anatomy and weight loss. You should be well-informed. Students will ask you questions, and you need to be able to answer them accurately.
Get certified in cardiopulmonary resuscitation. Health clubs generally require that their instructors have current CPR certifications. You'll need to get this renewed every year or two.
Look into fitness associations that offer primary fitness certifications. This is a general certification that covers you as an instructor. Most facilities will require that you have this primary certification. Two main associations that offer it are the Aerobics and Fitness Association of America and the American Council on Exercise.
Prepare for physical and written tests on your fitness instructor certification exam. Most certifications involve a one-day workshop that culminates in these two exams. The physical exam requires that you briefly teach a group.
Determine the type of fitness classes you'd like to teach. Don't feel that you have to limit yourself to one discipline. Many instructors teach a variety of classes.
Look into getting certifications in the particular disciplines you wish to teach. Many programs offer certifications in such niches as cardio kickboxing, indoor cycling, yoga, Pilates, core-body work and body sculpting.
Accumulate a fitness music library. You'll need lots of music to use in your classes. Start making mixes now.
Inquire at local gyms to see if they are hiring new instructors. If they are not currently hiring, see if you can get on their substitute instructor lists.