As an HS student, you will find difficulties to shadow a physician. Sometimes, it is even difficult to find shadowing when you are in college, the most definitive indication of your interest is after you took MCAT, at that time you will have less problem to find shadowing. The best way to get involved right now is to get a CNA certificate and start working in the hospital part-time or in those elder care facilities.
Unless you want to apply for BS/MD program, its too early to think about medicine until after you took some premed courses with good grades and decided medicine is for you, NP, PA, or MD. The majority of the first year college students gave up medicine after one or two years in college.