Seems like everyone here wants to give you crap just because you are showing interest in your future. Forget them.
First thing you need to do is do well in high school. Grades, ECs, SAT, etc... do as well as you can in all those category. Although you don't need to go to a "top tier" undergrad to get into medical school, it certainly helps to graduate from a well-known institution.
When in college, do things that will expose you to medicine, such as volunteering at a hospital. Make sure you know what doctors do, as well as the system that they are operating in (e.g. learn about HMOs, medical malpractice, residencies, etc.)
Do your best in school to get good grades. The #1 important thing to a medical school application is one's GPA, followed closely by the MCAT (an admissions test that all aspiring doctors have to take). Take a look at some of the extra curriculars that people say are important - research (biomedical, if possible), volunteering, pre-med organizations, etc. But most important is to remember that there is no particular mold that one has to fall into when applying to medical school, so you should do the things in college that you want to do. Don't like your pre-med organization? Don't join it; it probably sucks anyway. Do things that may not relate to medicine if they interest you, such as sports, mock trial, student govt., etc.
When picking a major, choose something that interested you in high school, or go in undeclared. You dont need to major in biology or bioengineering to get into medical school. In fact, non-bio majors generally get accepted into med school at higher rates, so take a look at something that doesnt follow the usual pre-med choice (like Political Science
) However, if you find biology particularly interesting, then by all means, major in biology.
Good luck with your ambitions. You don't need to stress about anything at this point, but don't let people tell you that it is too early to think about your future.