CSCS(Certified Strength and Conditioning Specialist) which is through NSCA must have a four year degree to take test though. Most gyms don't require any certification or just have you get a bs online certification for insurance purposes. The ACSM, NASM, and NSCA are only ones that hold any weight with any legit company or job. If you just want to make extra cash just train people at the gym you already use. As long as you aren't booking multiple clients in a row and hanging out there all day it probably wont be an issue. But plenty of people look good and don't know shit about actual program writing and specific needs of individuals. I see plenty of idiots training people that have no idea what they are doing. Not saying this is you, but if you don't have real knowledge don't take peoples money and have them do a cookie cutter routine or just what you do. Big pet peeve of mine.