Well here's the basics to get you started:
A fairly universal set of prerequisite courses in the sciences (Bio, Gen Chem, Org Chem, Physics, often Calculus and Biochem) to prepare you for the first two years of medical school
A truckload (pun intended) of clinical experience - hospital volunteering, physician shadowing, etc. (so they know that they're not wasting a spot that is going to go to waste on someone who is going to bail at the first sight of bodily fluids and solids/semisolids.