If you school has an MD/PhD program then it most likely will allow you to apply for admission to the program internally. However, from what I've read in the MD/PhD forum PhD candidates typically aren't accepted.
If you still want to try though, you'll need to take the MCAT, have the med school's pre-requisite courses met, be in your first two years of grad school, and I'd imagine you'll need to be roughly on par with the average successful MD/PhD applicant for your program (most likely something in the neighborhood of a 3.8 undergrad GPA, 36 MCAT (95th percentile), and 3 years of research experience). If this is a new decision then I'd imagine you haven't taken the MCAT yet which is going to be a considerable problem since I can't imagine trying to prep for that test on top of grad school.