I agree that it’s definitely best to be certified before you begin teaching classes. Most quality facilities will require it. As has been suggested, if someone is injured in one of your classes, your certification will significantly reduce the facility’s potential exposure. More importantly the certification will require that your professional skill and expertise have been challenged.
Rob- Yes do get certified but before you do that take as many classes as possible from many different instructors to see the different styles of teaching. Do Do Do shadow someone and see if you can eventually co-teach with your mentor. I suggest a smaller class where the participants are consistent and will be understanding with you learning. The certification will help you understand the differences between teaching a group and a individual. One thing you will also need to get if you are looking to get certified, is your CPR certification, do you have that yet?