Do you have a local YMCA? You can start by attending the classes. As you get to know the teachers, tell them you're interested in teaching. A good teacher will give you pointers, and she might even introduce you to the aerobic coordinator. This is how I got started, and I couldn't be happier!
I don't know about the other clubs, but the Y often has wonderful staff. A happy, supportive staff provides the perfect gym culture for a beginner instructor.
Keep working out to Cathe, she's the best!