For classes outside the College of Social Work, check the course catalog for any additional sections that might have a seat available. If there are none, enroll in a section with a seat on the waitlist. Another option is to contact the instructor of your desired section, and ask if she/he would grant you an override for the course (see below for further instructions if you obtain an override).
For social work courses that are full, check to determine if there is another section available. If there is no other section, email your advisor to determine if a seat can be opened.