I just sat in on a class at UCDavis. They had a 1L waiting for me when I arrived who walked me to class, showed me where to sit, and walked me out after class. Maybe thats what they have in store for you? If not, I would do exactly what the others have suggested: arrive early, introduce yourself to the Professor, ask where to sit, perhaps introduce yourself to the people you're sitting by (chances are they will be very helpful and can answer any questions about the school you may have), and enjoy!