You can major in whatever you want! That's the great thing about a lot of professional programs- as long as you have the right pre-requisite courses, schools don't care what your major is. You'll find a lot of Pre-PT students major in Exercise and sports science, or some variation in that, because it tends to be the major with the most relevance to PT, but totally up to you. That was my major and A&P were by far my favorite courses in college. You could also consider biology if you like A&P. Whatever you choose, just make sure you'll have time to finish your major AND all the pre-reqs, which can be challenging if you choose a non-science major. Take some time to look through the program requirements on PTCAS.org and see what kind of classes grad schools are looking for. Normally it's some combination of a year of A&P, year of Bio, year of chem, year of physics, up to a year of psych, semester of stat, and then some english, some math and so on.
As for shadowing- definitely go for it to see if PT is the career for you, but be aware that I'm 90% sure anything done in high school CAN'T be listed on your graduate school application, so don't go crazy with your hours, only to discover you can't list them. That being said, it'll still be a valuable experience to help you decide if you're on the right path
Good for you though, planning ahead! If I'd known I was going to apply to PT school when I was still in high school I would've done my first year of college verryyyy differently. Good luck in your senior year!