You should study carefully departmental requirements before selecting courses. Courses numbered on the 100 and 200 level, especially those which are prerequisites for more advanced courses, often have large enrollments. Class size in such courses can range as high as 500, although 100 to 200 students is more common. To reduce size, many of these introductory courses are taught in sections that cover the same material but have different instructors and meeting times. The presence of teaching assistants in such courses should not deter you from getting to know the professor.
Most professors welcome contact from students.
Always feel free to see professors and teaching assistants during their office hours. 500-level courses are graduate courses that are also open to upperclass undergraduates. Be sure that you have the prerequisites for any upper-level course you select. If you are in doubt, during Fall Orientation week you can talk to the professor who teaches the course. Courses numbered beyond the 599-level are not open to undergraduates.