I'd honestly wait, you got 4 years of college, 3 of which can be devoted to volunteering/shadowing/research/good grades/drinking ... I know its exciting to start out, but its a long process and I would strongly suggest you enjoy your last real summer of freedom. I'll be honest with you, although summer break in college is good, my summers pre-college were 200x better - no one worked any serious jobs and there was no responsibility, so people would hang out all week long, graduation parties that lasted almost until the start of freshmen year etc. Once you start freshmen year, try and pull A's obviously and then once you have a handle on the change of life style college brings, start volunteering. I didn't start until after freshmen year, and I got in straight outta college.
edit: to add, you probably could throw it on your AMCAS application, but a lot of people including my college advisors said that anything you did before college is nice, but should not really be emphasized in a personal statement, activities etc.