A gym might not be the right setting for you. To keep things personal, look for a trainer who can come to your house if desired.
After reading reviews, make contact with several trainers in your area. Schedule meetups with them to interview and get a sense of their training style.
Among the most widely recognized associations are the American Council on Exercise (ACE), National Academy of Sports Medicine (NASM) and the National Strength and Conditioning Association (NSCA).
The trainer you choose should know exactly how to avoid injuries and help you work out safely if you have other injuries. Make sure your trainer is first aid and CPR certified.
A personal trainer is just that—personal. It's very important you get along well with this person, so meeting up is critical for assessing your rapport.
After you have introduced yourself to your prospective trainers, see if you can schedule appointments to sit in on some trainings. Seeing how they work gives you a better idea what to expect.
Ask for their rates and compare them with your budget. Trainers sometimes have special promotional rates or packages that help you achieve your dream body without breaking the bank.