I don't work at 24 hour fitness, but am currently finishing up my studying to get certified and work at 24. So, I've been doing some research and a friend who works at a 24 in Sacramento recently just got a job there.
He's not certified and isn't pursuing a degree in exercise or any related field. He just works out a lot and knows his stuff. The hiring manager offered him a job and sent him home with a CD/DVD. It took hime about a day or two to watch, study and take notes. Then went back and took a 50 question multiple choice test. Passed and is now a trainer.
He makes $12.50hr as a personal trainer. If he gets what I'd call a 'real certification', one that requires months of studying, not a day or two, he told me you get a $6hr raise. You also get an additional raise on each additional certification, but obviously there's a cap and it goes on case by case situation.
Also, he mentioned you get commission on all the supplements/gear you help sell.
Hope that helps anyone else looking. Just walk up and ask, that's what I did before...most trainers and managers were cool with me when I was inquiring and stating that I was interested and studying for the job.