What is the best major for someone wanting to be a physician's assistant?

Any major is fine but look up the PA schools you are interested and take a look at what pre-requisite courses are required for admission. Biology and other sciences are typical choices because they cover many (but not all) of the pre-reqs for PA/Medical schools. You can of course major in anything as long as you take the required courses that may not be a part of your major to enter PA school.

And to answer your other question, you will need many volunteer, shadow, and/or work experience the medical field. Start volunteering now, and ask around local clinics and hospitals if you can shadow!

There are many major's that will help in becoming a physician assistant. Biological Science is probably the broadest major. You could do Neuroscience and Behavior, Chemistry. The key is the courses in that major. You will have to have Anatomy, Physiology, Organic and Inorganic Chemistry, to name a few.