Choose your undergraduate major. Contrary to belief, a pre-med can major in absolutely any major they wish. The only thing required for medical school is certain pre-requisite classes: one year of general chemistry, one year of organic chemistry, one year of physics, one year of biology, and at some schools, one year of calculus and college composition as well. Once you've declared a major, move on to the next step.
Get ready for quite an academic challenge...the pre-requisite classes for medical school - particularly organic chemistry - are challenging. They are not courses one can breeze by, and extra care should be made in passing these courses with high marks. The admissions committee at most medical schools judge the pre-requisite classes quite highly. Start arranging your pre-reqs your freshman year: you need to get the ball rolling early. The reason is, it will take at least two years for the pre-reqs, and you apply to medical school at the end of your third year in college - leaving the extra year for core classes, studying for the MCAT, and, well, having a life.
Step up to the big beast that is the MCAT. After the pre-requisite classes are done, it comes the time to study the MCAT. That is, the medical college admissions test. It is a very hard test - some say the MCAT is the most important test in a soon-to-be doctor's career. It does, after all, weigh a lot towards your future acceptance. I recommmend that all pre-meds grab themselves a copy of the current Medical School Admissions Requirements (MSAR) available on amazon or from the AAMC directly - it will give you the current averages for GPA and MCAT scores.
Start begging your professors, that probably don't remember you, to write nice things about you! That is, gather letters of recommendation, and then, apply to medical school. You should gather letters of recommendation from science professors, people you've worked with, etc. Anyone that can speak highly of you. Once you have these, you apply to medical school using the AAMC's AMCAS system. You have to pay fees for each school you select, and often they send secondary fees as well. Pray that you get an interview invite!
Sell yourself! Once you get an interview, learn how to make yourself look like the best thing since sliced bread. Make sure you have practiced over and over your interview skills. Impress the admissions committee and make them feel like they absolutely *need* you at their school. The more interviews you get, the better your chances of getting accepted. After all your interviews are over, sit back, relax, and..well, pray. Good luck, future doctors!