This article is from the Medical Education FAQ, by firstname.lastname@example.org (Eric P. Wilkinson, M.D.)with numerous contributions by others.
Your primary source should be your college's pre-medical advisor.
Make an appointment with him/her early on--sophomore or junior year
would be best. Make sure he/she pulls your transcripts, etc. before
you show up. The two of you can talk about your strong and weak
points, what you could do to boost your chances, and which schools
you should apply to. Also keep in mind that most pre-medical
advisors send a letter along with your applications, so getting to
know him/her will help get a more accurate letter for your file.
The Internet is a good source. Most medical schools have web sites
that give lots of information, application requirements, etc. In
addition, post any questions, concerns, fears, or despairs to the
misc.education.medical Usenet group. It's populated by lots of
grizzled veterans who have been through this process (sometimes more
than once) and can help you avoid the pitfalls.
Another essential source is the MSAR (cf 1.4).