I wanna be a personal trainer!!!?

im going to college for dentistry, but ive been going to the gym for a couple years now and ive done tons of research over the years so i have tons of knowledge about fitness. I know for a fact i have wht it takes to be a personal trainer. Someone said you just take a test and if you pass you can be a certified personal trainer? is this true. if not what do you need to be one bc i would love to do that as a side job.