Have you taken many fitness classes? What types do you enjoy? Find out from instructors in that field what their licensing and certification is. If you want to teach group classes, most places will hire you with a basic certification like AFAA group exercise and then a specialty certification in whatever specialty you prefer (i.e. if you want to teach a branded format like Zumba, you need to take a course and pay the licensing fee to use their name.
Some specialty formats, including Zumba, will allow you to begin teaching without having a base certification. Personally, I would NOT recommend that. There are safety, programming, and anatomy/physiology topics that are not covered in specialty licenses.
[Don’t get me wrong, I love Zumba and I teach several of its formats, but I also have quite a few certifications under my belt, not just a Zumba license]
Best of luck to you!