Pick a major that interests you so you don't mind devoting a majority of your hours to studying. You will need to get good grades in college in order to apply for medical school. At the medical school I attended, the average GPA is reported to be 3.85, so even one or two B's can hurt your chances of acceptance.
Aside from this, any major is acceptable as long as you complete the prerequisite courses.
Typical medical school prerequisites include:
Biology: Lecture – 4 semesters; Lab – 1 semester
General Chemistry: Lecture – 2 semesters; Lab – 1 semester
Organic Chemistry: Lecture – 2 semesters; Lab – 1 semester
Biochemistry: Lecture – 1 semester
General Physics: Lecture – 2 semesters; Lab – 1 semester
Math: Statistics – 1 semester
English: Rhetoric (Composition) and Literature – 2 semesters