The catch with being a personal trainer is 9 times out of 10, it's not very lucrative. The better money is in private practice, but it takes time to build up the clientelle.

If you're looking to get on at a corporate gym, the best bet is to try and work your way up to being either a sales manager or fitness manager.

Also, with a lot of gyms, it's best to get your certification out of the way before hand. Most places only require the basics, like and ACE or APEX course. Fairly cheap, and you can always go for the better certs later.