Ok, well like many people have said, if you do dental hygiene you won't be getting a college degree, thus you wont have a major. You will go to a community college or what not and be done is about two-three years, probably with an associates in dental hygiene. They do offer bachelors' degrees (what most people call college) in dental hygiene, but those are not as prevalent and probably not associated with the typical college experience.
For dental school you will need a bachelors' degree with a major in whatever you'd like. Most people do biology since it covers most of the pre-req's. You will have a to take quite a few science courses no matter your major (for the pre-req's). Then after the 4-year college degree, you will have a 4-year dental school education. Dental school is hard to get into, but, on the other hand, it's not something you can't accomplish if you put your mind to it.
You should shadow some dentists, and dental hygienists while you're there, and see which you like. Talk to them about each profession.
Also, you say you want to be a dental hygienist, why do you want to do that?
Edit: Use Google, that's why it's there. It will give you some basic information on each. You can Google about each career, education, difficulty, what they do with their job, professional websites (like ADA & ADHA), etc. If you are truly interested in these careers you should invest the time to learn about them.