First off, I've been where you've been. Now, I'm leaving in January for community health development in Burkina Faso. There are LOADS of things you can do to make yourself more competitive. And you can start now!
I tutored younger kids when I was your age (volunteer and paid) and that was a good start.
Next thing I think is good to understand is that the Peace Corps accepts people from loads of different backgrounds. Get your college degree in whatever you want! There are some degrees that will lead to more specific Peace Corps placements (forestry, nutrition, agriculture) but I have a degree in Political Science and will be doing health education.
I did study abroad my sophomore year of college. I went to London and had an internship in Parliament because it's what I'm interested in. So, I didn't get the language immersion but I also started taking French when I was a sophomore. Foreign language is important to make you more competitive, and as I understand it, to make life easier in service. When I got back from studying abroad, I got a job as an intern with my school's international education department. And then I got a job as an intern with an after school program with at-risk youth. I did activities with them and tutored them in all subjects.
When I had my interview with the Peace Corps (This is under the old system so the interview actually came first when I applied), my recruiter told me I would be competitive if I volunteered for at least 20 hours a hospital. So I did that as a senior in college.
So the take away: volunteer with anything you're interested in (don't force it, you have loads of time), get a degree in whatever you're interested in, study abroad, learn a foreign language.