Find out the specific unit requirements of your college. It's important to know how many classes are requirements and how many are electives.
Thumb through the course catalog and make a list of courses that interest you. Divide them into 3 categories: major, core and elective.
Choose core classes and classes for your major first, as these offer the least flexibility. Consider meeting with your advisor to find out which core classes you must take for the major or majors that interest you.
Choose electives once you know how many elective units you can take and once you've chosen core and major classes.
Ask older students or your advisor about the professors who will be teaching the courses you're considering. Even the most fascinating-looking material can put you to sleep if the professor can't teach.
Sit in on more classes than you'll end up taking, for at least the first few days of the term. This way you can see what you like before committing for an entire term.
Drop the classes you don't like or need, and continue attending each class you find interesting, even if you are not yet enrolled. Persistence can pay off.