Not too difficult at all, really. The major concern is research experience, but you can get involved in scientific research regardless of your major. You will probably need to get your science courses out of the way quickly, and it would probably help if you took a quantitative chemistry course, but I'm sure there will be plenty of faculty members at your school that will have no problem taking a non-science major into their labs. You may have to convince them that you know enough--but fulfilling your science requirements will probably do that. Also, you may have to talk to a few professors before you find one that is willing, but if you're determined, this shouldn't be a problem.
Obviously, you should take general chemistry and general biology as early as possible. The others can wait until after these two are completed. This is assuming you are interested in biological or biochemical research. If you are interested in something exotic (for an MD/PhD applicant), like geophysics, then you may want to take physics first.
