Hello!
How far along are you in undergrad?
The usual advice is to get both GPA's up as high as possible, with the very minimum being 3.0 in both areas (overall and sci gpa). Something to consider, if you haven't already, is going to a D.O. school instead of an M.D. school. Check out the Pre-DO forum, especially the FAQ thread near the top.
The upside to applying to D.O. is that your retake grade will be the only grade considered for your GPA... that is to say, you can turn your C's and D's into A's and watch your GPA improve drastically (a lot easier than doing it for M.D. schools, which will just average your grades for your GPA... for instance, a C and an A is about the same as having a B).
Anyway, if you're obsessed with becoming an M.D., your options are post-bacc (I don't know how adcoms look at informal vs. formal), SMP (Special Master's Program, at places like Boston University and Georgetown), or some other Master's program. Understand that doing any of these does not necessarily erase a "poor" undergrad record, but supplements it. Hopefully adcoms will look more at these grades as being indicative of your academic acumen.
Good luck to you!