Just call your local hospital or look at their website. Make sure you let them know that you want direct patient contact when you volunteer, so you don't get placed in the kitchen or shoveling snow. I volunteered in the pediatrics unit of a hospital and I got to play with the kids in a play room and the sicker ones on their beds, and I loved it. It was through the child-life program is you're interested in something like that, many hospitals have it.
Like someone else said, Med schools want to make sure you can handle sick people and you know what you're getting yourself into. My undergrad institution recommends at least 50 hours, I did about 85 because I had such a great time. When you're doing your volunteering make sure you get to know a supervisor or someone who can write you a letter of rec! I can't say because I'm not on an admissions committee or anything but I would think volunteering would almost be a requirement to matriculate.
Good luck and find something that you like...hospitals are big with many tasks you can do in many departments!