Qualifications
You'll need to attend one of the 330 Accredited Dental Hygiene programs approved of by the CODA (source).
You can usually do this in between two and three years, depending on whether you go to a junior college and get an associate degree or not. There are stand-alone programs, online programs, and even programs in four-year universities (source).
You'll then need to pass your dental hygienist written boards and your state exams. After that, you'll be licensed to practice through your state agency. You'll also need to pass a CPR exam, and you'll have to get your school transcripts, as well as letters of recommendation from dentists in the area who would be willing to hire you (source). Then you can be a dental hygienist. It sounds like a lot, but trust us, it's much harder for dentists.
You'll need to be good at math and science in high school and college. If you're math-phobic or can't handle heavy-duty anatomy, physiology, nutrition, and chemistry classes—this might not be the right job for you.
You'll also need to be good with your hands, and we don't mean in the masseuse kind of way. You'll be using tools and scraping hard decay for hours on end, so you'll need to be dexterous, have good hand/eye coordination, and great stamina (source).
You should also be personable. You're going to have to get to know your patients and talk to them if you plan on having any lasting relationship with them. Nobody wants to spend forty-five minutes in a chair with their mouth opened in the company of someone who doesn't say a word to them or who likes to hum show tunes through their nose. So that's at least one habit you'll have to break.