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.
This means you attend it without being registered and attempt to get in when others drop the course. Also, find out how long you can remain undeclared. You don't have to choose a major right away. If a lousy professor is teaching an interesting class, find out when it will next be offered and who will be teaching it. It may be worth the wait.