High school grades are irrelevant to (just about) everything not related to college admissions. For example, I had a rough time in high school chemistry and physics yet was the top student in every chemistry and physics course I took. Many incoming freshmen have difficulty adjusting to the demands of college courses and end up dropping out or doing relatively poorly during their first year. Don't let that discourage you, though. As long as you study effectively and often (almost every day), you will be fine. People who do poorly usually expect college to be like high school and end up not putting enough effort in, not to mention that many freshmen are emotionally immature.
At the moment, the MCAT should be the least important thing on your mind. If you want to be a pre-med student, fine. Just be sure to choose your college adviser well (if your school permits this) and get to know the pre-med adviser and be familiar with how your university handles pre-med committee evaluations and the like. At the beginning, you should also be focused on volunteer work, which should be done in both a hospital setting as well as something not related to medicine at all. Be consistent and stick to as few organizations as possible and work as often as you can without ruining your GPA and social life. A healthy social life is something you should do to keep yourself happy.
EDIT: One last word of advice, you should get to know your professors. Professors tend to remember friendly, bright students for years, which is what you need when it is time to get them to write you a letter of recommendation. It also opens up opportunities for research, which you should definitely look into if research is of any interest to you. From personal experience, research has tremendously helped to integrate and add to the knowledge I obtained from my classes. You'll also get to see some cool things if you get serious about it.
Also congratulations on entering an exciting phase of your life. Be sure to approach everything with an open mind and enjoy your college experience. Once it's over, it's over. You will not be doing yourself a favor if you neglect the little things. Even though your future career is important to you (indeed it is very, very, very much important) you need to remember it is not #1. Happiness and health are always higher priority than grades.