NSCA and ACSM.
How far do you want to go with this? If you just want to be a personal trainer, go for NSCA-CPT or ACSM. To go beyond, try to get yor CSCS from NSCA. I'm USAW-Certified and I'm going after my CSCS. I'm not willing to become a just another personal trainer, though.
And why post this in the "Supplements" forum...???