Chance of Acceptance into Medical School

Hello I just finished my freshman year at the College of William and Mary. First semester I got a 2.65 (with a C-in Gen Chem and C in intro to Bio). My second semester I did much better in my science (classes B in Orgo and B- in Bio), but do to my lack of participation in the my government Fresh Sem. i earned a C-. I plan to start research next semester, working in a clinic in South America the summer of 2011 and do a shadow program during my Junior year. I was wondering if my mistakes freshman year will not allow me to gain admission into a top 20 medical school. Can I please get some advice as to how I should go about the rest of my college years to optimize my opportunities of getting into the school of my choice, I am willing to do whatever it takes.

Thanks,
Pre-med in VA