You ask a great question and I am starting to teach before I receiving my certification. I am certified as a personal trainer, so that helps. But I have also been a teacher and a coach.
If you want to start teaching before being certified, make sure you get any specialty format certifications (if there is one for step and the kind of aerobics you want to teach) since that will help with your skill set. Also, as you practice, you’ll build confidence. You can also practice teaching your friends and family before going in for an audition.
As you prepare for your exam and work on your routines, recall every way that you may have taught others how to do things and how you were taught to remember your moves and that will help. Most employers want to see that you have the energy, motivation, and ability to teach the class. Not all group instructor certifications give you that practical experience, so you have to work on those skills as often as you can.