Look online. The Harvard University website has information on all their courses, as well as admissions, enrollment requirements and even financial aid (see Resources).
Call the Admissions office at 617-495-1551. You can call and speak with a member of the staff, or give them your address to have them send you an information packet.
Mail a request for information to Massachusetts Hall, Cambridge, MA 02138. You don't have to send a specific form; you only need to mail a regular written or typed letter asking for information.