You will first need to put in an application to that school. In some schools, particularly those that are parts of standard universities and colleges (even schools like Harvard and Requirement use E Degree programs.) The application procedure will take simply a couple of minutes, often done best online.
World-famous Harvard University currently offers 42 of its classes for free online. Many of these classes on edX, like Introduction to Computer Science, are self-guided and ready to start anytime you’re are. Others, like AnatomyX, run on a fixed schedule. Some even offer college credit through the Harvard Extension School for a fee; otherwise, completion earns you a nifty free ‘Honor Code’ certificate. You can browse Harvard’s online course listing at edx.org/school/harvardx.