You can usually find classes to be a personal trainer at your local gym.  They often give classes in this field in order to induce people to work for the gym.  People who become a personal trainer usually start out by working out at the gym.  Because people see them there often, they start asking them for help with equipment.  A good personal trainer is naturally friendly and helpful and may start actually training people at the gym without even being hired!  Once they realize what they are doing, they can approach the gym management and offer their services.  Some people do it in exchange for free gym membership while others make a lucrative living as a personal trainer.  If you build up a steady clientele, you may be able to make this your full time business.