If you are taking a course that has a a course end date, this is most likely going to be a live and moderated course. The best way to contact your instructor or his or her teaching assistants in this kind of course is to post a discussion in the Discussion section of your course site, and add "[STAFF]" to the subject heading of your post.
This enables them to easily find discussions addressed to them and respond accordingly.
In some cases, the instructor or teaching assistants will offer supplemental contact information, in which case, feel free to additionally use these other alternatives. If they have, typically this information will be displayed on the course's home page or a class syllabus.
If you are taking a course that is listed as self-paced or ongoing, it is most likely going to be an un-moderated class. We recommend using the forums as a way to get help from your peers in the course.