IF A CLASS IS FULL
First, on Saturday, September 3rd, upper-class students begin to add and drop classes when registration opens online for all students. You should get back online to see if space opens in a class that you want and hopefully it will.
Second, on the first day of classes (Tuesday, September 6th), look again for openings online as students will continue to add and drop classes. This is the start of the Change of Program period, which begins on the first day of classes and lasts until Friday, September 16th.
Third, if you still want to get into a class that is full, plan on attending the first meeting of the class when instructors typically announce their over-enrollment policies. Bring with you an ‘Add/Drop’ form and hopefully the professor will sign you in over the class limit (http://registrar.columbia.edu/files/registrar/registration%20and%20add_drop_1.pdf). In the end if a spot does not open up plan on taking the class another semester. In the meantime you should be attending the next best option on your list of classes so that you are not caught with too few classes.