the best thing to do is to try work at a gym where you can work as one. you will probably have to split the money with the gym (only like 5 or 10 dollars per customer) Then after a while you can get people to work with you without going through the gym. That's the best way to get started. Hope that help!!
Or you could work in a nutrition store and get your cleintelle there, people walking in are alot of times totally clueless. The hardest part is getting the clients.
Good luck and be patient - you will go through alot of clients who really just don't care. They want your help - you design a plan and then they make it a week and drop out.