What a great question! It's so cool that you are already thinking about how to get a head start...that's a very smart thing to do as it will make you competitive when applying for college.
I believe the technical term for your field of study is called Forensic Psychology, I suggest you start by heading to your library and reading anything you can get your hands on (this should be easy for you since you clearly have a passion!) If you're lucky enough to be in a school that offers Psychology coursework- take every class you can. Taking Sociology classes won't hurt either. Sadly, finding volunteer work that focuses specifically with criminal minds/ behavior will be hard; but you might consider offering volunteer hours at a local general counseling center. That would at least be experience in understanding how offices run and will afford you the added bonus of networking with people in the field (networking is one of the most important aspects of career development)...plus Psychologist/ Counselors/ Social Workers tend to really enjoy people and are often on the cerebral side...so often times they enjoy when people "pick their brains."