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Ann's answer is right on the money. I'd just like to add that if you like your Criminal Justice major, than by all means keep studying it! College is a great chance to learn more about yourself and explore things that aren't medicine related (you'll have plenty of time to study medicine in medical school).
As long as you can fit the premedical classes into your schedule, the only reason to switch your major is because you've decided it's not for you.
You don't need to have any specific major to apply to medical school, but you have to be sure to take all the pre-requisite classes. Many students major in the sciences, because most of the pre-reqs are covered within a science major, however, you can actually major in whatever you like.
For most school the science pre-reqs are:
2 semesters general chemistry with lab
2 semesters organic chemistry with lab
2 semesters biology with lab
2 semesters physics with lab
1 semester biochemistry (not always required, but becoming more common)