It is not absolutely necessary for everyone however, the impression I've gotten from everything I've read on SDN is that it is pretty much a pre-requisite for top tier schools. If you're worried about ECs focus on shadowing and volunteering first, then research.
Getting a research position is easy if your school is a research institution. Just look on your schools website to find out what professors are doing and ask one of them if you are interested. They will expect you to have at least some knowledge of the field. For example, do not expect to enter a cell bio/molecular bio lab without having taken and done well in cell biology. As far as your involvement, you will start off tagging along with grad students, they'll teach you some stuff, if you do well you'll gain more independence. Chances are you'll be washing at least some glassware or stuffing pipette tip boxes. It's just part of being in a lab, everyone starts there. You should expect to be doing more than that eventually though. If you aren't really doing much, then it's worthless as far as med school is concerned because you will need to be able to talk about what you did. Washing dishes and taking notes won't get you far.
If you aren't at a research institution, talk to a professor you know well and see what they have to say. Usually they will recommend something to you, maybe they even have research going on. Sometimes at small institutions like the one I come from, they will even come up with a project for you if they aren't actively doing something.
Good Luck!