there are many organizations through which you can become a personal trainer, and you can find that here at IDEA http://www.ideafit.com/fitness-certifications.
Which one to choose is the most frequently asked question, and I would look for one that is NCCA accredited and nationally recognized. My recommendations are usually for ACE, NASM and ACSM. I am somewhat partial to NASM. They have developed a model for training called OPT which is re-iterated in every of their specialty certifications and thus creates a coherent unit.