Start by doing well as a freshman in college. Take Gen chemistry and Bio and do well, you want to maintain at least an A- average in the sciences. Move on to Organic chem sophomore year, then physics and biochem. Also take humanities classes like psychology and sociology.
If debt is a concern, attend state school or apply to schools that might give you a scholarship. If competitive, high ranked private schools will cover cost of tuition through need based financial aid.
Take the MCAT at the end of summer after your sophomore year.
Good luck!