You can get information about the physics major here. Once you are ready to declare the major, select a major advisor from this list. Visit you major advisor, or ask to arrange for a video meeting, and complete the Declaration of Major form via DocuSign (Instructions here).
A declaration of a BS Astronomy/Physics major takes another step. Your advisors will always be the Directors of Undergraduate Programs, currently Michael Skrutskie and Stefan Baeßler. You need to arrange meetings with both. Then, use the older form here, collect signatures from both advisors, and send it to our receptionist for further processing.