Getting into Medical School starts well before you sign that application. Preferably planning will start in high school. Getting the best possible grades will help your admission. Having a well rounded portfolio will also help get into medical school. Grades are important but being a well rounded individual can be just as important if not more so.
Volunteering at hospitals or as an EMT shows that you are committed to the health field and that you have a serious interest. Doing well in college is a given for importance. Study up on the scientific fields and in your junior year take the MCAT which are the entrance exams for medicine.
Once you get your scores back start applying to the medical schools of your choice. The final step is prepare to interview. The entrance interviews will either make or break your acceptance. Be yourself and dress appropriately for the situation.