Ask yourself the following questions before choosing a major:
1. What do I enjoy studying the most?
You only go to college once. You may as well study what you want, because you may not get another chance. Take a variety of courses your first year so that you can figure out what excites you. The more you enjoy the subject you're studying, the more you likely you will be to devote yourself to your studies and get better grades.
2. What am I good at?
The counter-argument to the above - passion for what you study is overrated. What matters far more (at least, for getting into medical school) is whether you are good at what you study. If so, you are more likely to get good grades. Professors are more likely to notice your abilities, opening doors to other opportunities. And your studies will require less time, allowing you to have a life and pursue opportunities outside of the classroom.
3. What is a subject that might give me a marketable skill?
A marketable skill is something that someone would pay you to do. The most basic example is web design. If you're able to design a website, you will be much more likely to be hired (either part-time or full-time, depending on what you want).
What does this have to do with applying to med school? Well, let's assume that you decide that you don't want to be a doctor. I know you can't imagine that, but it happens all the time. What will you do with yourself while you figure out your next step? It helps if you have a skill that someone will pay you to do, to help you earn a living or get that first job. A marketable skill can also be helpful while taking a year off preparing for med school, because it can help you support yourself and earn money for the applications and for med school.
4. Is this major something I want or something my parents want?
Your parents may be helping you pay for college, but you're the one who has to study the material. While your parents have your best interests at heart, you may need to have an honest discussion with them about what you want to major in and why.
5. How is the department at my college for this major?
Are there lots of resources for the students in this major? Opportunities to do research? Lots of classes in this major? Not sure the answers to these questions? Talk to other people who are currently majoring in the subject and see what they say.
Still not sure what to major in? Take your best, educated guess. It's fine to switch majors, so if you start down one path and decide you don't like it, you can switch.