Complete the required pre-requisite classes for the MCAT (one year of chemistry, physics, biology and organic chemistry). Not much detail will go into these classes in this article. The first step in studying for and passing the MCAT is to get a hold of a good set of MCAT study books. My personal favorite is Exam Krackers - they offer a nice, full set as well as audio recordings (Audio Osmosis). Other popular study sets include Berkeley Review, Kaplan and Princeton Review.
Create a schedule for success! Once you have aquired your study materials, the next step is to formulate a schedule that you will follow *religiously* the next 8 to 20 weeks or so (depending on how much time you have). The schedule should have you studying for the MCAT at least 3 times a week (at the minimum) - I recommend 5 times a week, just so it stays fresh. This is *not* a test you can cram for. Make your schedule so that there is time for breaks, and during which, make sure you actually take a break. It helps consolidate what you've just studied. After you've completed your schedule, move on to the next step.
Get prepared to become a hermit, seriously! Before the time comes to study the MCAT, explain to all of your close friends and family that you hang out with on a regular basis what it is you're trying to do. If you tell them how important it is not to ask you to hang out on X days at X times (according to when your scheduled study time is), it will make it that much easier to stay loyal to your schedule and not get sidetracked.
Study, study, study. Follow your schedule, and make sure to get the practice MCAT tests from the AAMC. Each week, you should take a practice exam under the exact situations in which the MCAT is administered. Monitor your progress and figure out where you are going wrong. You should keep repeating this process over and over until you are consistently scoring as high as you would like. This is key.
Relax - all work and no play is no bueno! Once you are all done studying, take a few days to relax and let everything settle in before the actual day of the exam. Not too long though - you don't want to forget minor details that you may need to know. Once the day comes: get a nice rest, have a good breakfast and arrive at the test in an optimistic mood. You will do fine.. and if not, well..you can always retake it! Good luck!