In the US, to apply to medical school, you need a bachelor's degree. Any 4-year university should suffice.
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
Education consultant. 10 years history as college instructor.
Any college advisor can help you plan the classes you'll need to pursue a medical degree afterwards. A good college program will PROVIDE what you need to know before you apply to medical school, so don't feel like there are things you need to know NOW in order to start striving for that goal. To get into a good college program, though, and to be successful there, you should take whatever science classes you can (especially biology and chemistry) and math. These will give you an idea what kind of topics pre-med students will study in college, and give you some fundamental knowledge that should help those college classes go more smoothly for you. It will also help you determine which of those classes you like best, and what topic you might want to choose as a major when you get to college.
I'm not a doctor, but I suggest you read "Osler -- Inspirations from a Great Physician," by Charles S. Bryan. And even if you choose not to pursue medicine, you'll learn a lot from Dr. Osler that will help you all of your life.