Surf this site to find a training and certification program in your state. You'll find a range of prices and program-lengths (ranging from two weeks to two years), so make sure you scout around for the course that best fits your needs.
Sign up for their $300 beginner course to start leading your own Spinning classes. (FYI: You'll also need six months of teaching under your belt to complete your certification)
The National Academy of Sports Medicine offers a CPT (certified personal trainer) exam for $550 that will prepare you to design workout programs for clients, assess fitness levels, and safely develop and modify exercises.