That is a great question considering other countries do not require a bachelor's degree before entering medical school. As a current medical student, I would take the time in college to explore career opportunities and activities with an open mind. While you may be interested in medicine now, your views may change as you enroll in different courses and participate in different extracurriculars. Take part in those activities with an open interest - you never know how your opinion may change after any given experience.
For the medical school path specifically, I would focus on learning how you learn (corny, I know). However, medical school will inundate you with information every day, every week, and every month. Take the time now to observe whether you learn better by looking at a graph or picture, listening to someone explain a concept, or by acting it out yourself. For myself, I have learned that flashcards and repeating information frequently helps me to best keep it in my head.
Lastly, college is a great time to be involved in all sorts of medical-related activities. Volunteer at a free clinic, shadow some physicians or work as an EMT. There really are endless options for being involved in the field. Not only will this confirm or deny your interest in medicine, but it might help you focus on what type of medical specialty you are most interested in for a career. The application process is competitive and it gets more competitive every year. Not only does college give you the time and the resources to participate in these activities, but it also demonstrates to admission committees that you have a genuine interest and have put in the time to prepare yourself for the rigors of medical school and the lifestyle of the career.