Qualifications
Other than the educational requirements previously outlined (ninety-plus semester hours with science requirements, maybe a bachelor's degree, chiropractic school), there aren't really any specific requirements you need to meet. It'll take a lot of work, though, so you'd better be really into the science of spinal readjustment.
You'll also want some decent people skills, since all of your customers will be people, and there's a decent chance that they'll be skeptical about whether or not you can make good on your promise to help them.
The ability to make people feel comfortable enough to have you poke at something that really, really hurts is a great thing to have in this profession. So is the ability to convince them that you aren't secretly an old-timey witch doctor.