In personal training, one-on-one in a gym, you need absolutely no training, certification, or even knowledge to get started. It's a bad situation from the consumer's point of view, which I imagine means a bad situation from the point of view of competent, knowledgeable trainers:
http://www.stumptuous.com/how-to-choose-a-personal-trainer
Many trainers are fly-by-night, students trying to make an extra buck, people filling time till they get a “real” job. There’s nothing wrong with working part time as a trainer but often it results in people with minimal hands-on experience. If possible, find someone who has experience working with various types of people, and if you get really lucky, someone who has powerlifting or Olympic lifting experience.
The trainer I use now works at a small gym (where I pay $10 per month), charges $25 per hour, and gives me competent coaching on all the lifts I want to do, including powerlifting and Olympic lifts. He has a sports training degree and experience both as a high-level high school athlete at a school with a very sophisticated training program and also as an assistant trainer with a Division I college sports program. That gives him no economic advantage over the Brad-Pitt-in-Burn-After-Reading type trainers. If you go to a fancy gym and hire a no-nothing bimbo or himbo to train you, odds are they'll charge a lot more. Knowledge and enthusiasm for the job are cheap. If you want to make money in personal training, you have to sell yourself on the other factors: looks, rapport, motivational skill, etc.
P.S. Make sure you check out the sample job listing at the end of the post I linked above.
