Certificates are generally obtained through fitness schools. Of course, some of them are registered by the national association, but in general, it is better to study and take the exam again. A fitness instructor should not only know how to practice, in fact, he is a very systematic subject, such as nutrition, anatomy, sports rehabilitation and so on, many of which can not be learned by you alone in life practice. If you have time, you can go to a fitness training institution to study systematically, provided that you know more about several fitness training institutions and avoid being cheated.