Shadow your football or basketball team with the physical therapist of your high school to learn the different body injuries and curing techniques
Be sure that you are in active shape and very physical. Your classes in college will involve golf, tennis, swimming, P.E., ect.
Study the body parts and functions as much as possible.
Start looking for your internship your sophomore year in college to have it in line with your senior year, stay focused in school with your grades because it will impact your opportunities.
Try to volunteer to help out around the community with sports teams to get a feel of what you'll be doing as a career so that it comes to you naturally, because you will need to be a outgoing people person.