I'll try to answer, but it is a very broad question. There are a million factors that make a good candidate for acceptance, so here are what I think you should try to do.
First, and most obvious, do well in your classes. But please don't have a heart attack if you get some B's. I always read pre-meds who freak out and think they are going to be cast into the lake of hell b/c they got a B on a test or in a class. Also, you are young, so enjoy life too. Make sure you go out with friends and have a social life. Work hard, but don't be afraid to reward yourself too. As said before, do well on the MCAT's, but please don't ask what is the lowest score you need to get accepted. That is just a stupid and aggravating question, and a bad way to approach the test. Have a minimum goal in mind and if you can get that, then great, if not, try again...no big deal. Also, get clinical exposure. This is weighed more heavily than what people may suspect. Try to get certified as an EMT or have a lot of shadowing under your belt. There is no better way to prove to both yourself and adcoms that you want to be a doctor than actually doing something like that. Make sure that you shadow a DO if you plan on applying to osteopathic schools. You don't need 100's of hours, but a day or two is fine, just so you can talk to the doc to understand osteopathy a bit more. Volunteering is good to, but you don't need to go crazy either. If you want to do research, go ahead. Enjoy your classes and see where life takes you.
Oh, and if you can take a human anatomy course, even just as a summer course at a comunity college, do it. I couldn't imagine doing med school without having taken that class. It has helped me make the transition to med school so much easier and I, along with many of my classmates, have said that taking a human anatomy course in the #1 class we would recommend to pre-meds.