First things first - enjoy your senior year of high school.
Don't even think about the MCAT yet. A big part of preparing for the MCAT is learning the material in your bio, physics and chem classes as you take them. The real "studying" takes place a few months before the exam and consists of reviewing material that you hopefully already learned in classes and taking practice exams to build up your endurance and get used to the style of questions.
Honestly, the only thing I would recommend now regarding the MCAT is to get in the habit of reading. Read for fun and every so often read articles from the New Yorker or Atlantic Monthly. If you are used to reading this type of material, the Verbal section of the MCAT will be much easier.
A big part of undergrad success is learning to keep your goals in mind, but focus on what needs to be a priority now. So keep the goal of med school in mind and in August focus on starting off freshman year on the right foot. For now- just enjoy life!
Oh - and in terms of advice for college.....work hard, play hard, don't do anything too stupid and try not to get arrested. Most importantly, don't take too heavy of a courseload for your first semester. Allow yourself some time to get used to the whole college experience. Start looking for activities which interest you -research, student groups, EMT class - whatever you like. Keep a good balance by keeping your priorities straight - don't blow off your classes because you have to many other commitments, etc. Have fun and don't structure your whole life around getting into med school.
Deirdre