Dedicate yourself to doing well in all your courses, especially science courses, from your first semester. Many programs will expect minimum grades of C or B in prerequisite courses.
Become involved in a variety of campus organizations.
Become comfortable with speaking with your professors and advisors. Visit them during office hours to discuss your performance in class and future plans.
Begin to research PA programs to which you would like to apply http://directory.paeaonline.org/. Study their admission and prerequisite course requirements and tailor your courses accordingly.