Oh, the med school path. Good luck. I went down it. It's tough.
I'd start out with a lot of reading. A good place to start might be the Student Doctor Network. It's a forum (like this one) that focuses on medical school. People there can be intimidating, though. Get some books out of the library about what to expect in medical school. Make sure it's what you want before you actually go for it. It's a LOT of hard work, but very fulfilling if you commit yourself and feel called to medicine.
The best way to start out would be to talk to a premed adviser at your university. In general, though, the science courses that you need as prerequisites for med school are biology, physics, general chemistry, and organic chemistry (all one year with labs). Some require calculus too.
You can work the fact that you were a sociology major to your advantage for sure. Med schools value diversity, and now you have a way to stand out from "biology" majors. You have to sell yourself! In your application, make sure you highlight WHY being a sociology major will benefit you (different way of thinking, etc). Never rely on the med school to pick valuable things out of your application. POINT them out.
Med school admission is more competitive than (most) grad school admissions, so stay positive and strong! Go for it :-)