Go for the gold standard in the industry ether the National academy of sports medicine www.nasm.org or American college of sports medicine www.acsm.com That’s my opinion. I feel being certified by ether of these two lends a great deal of credibility to you as a profession trainer.
Best of luck!
You can begin by checking into Personal Training certifying organizations by conducting an online search. There are many organizations offering personal trainer certifications so you would be wise to take a hard look at each one’s requirements for certification, acceptance among gyms in your area, and cost (although I would NOT use cost as the ultimate determining factor here).
I hope that this helps. Good luck!