Discussions are a valuable learning technique; they ask learners to work with and think about the course content in active ways. As a result, students who participate fully learn more and also gain a sense of community through contact with others in the class.
Instructors actually appreciate it when students ask for clarification, since if one student doesn't understand, others might not either.
You waste your time and the instructor's by asking questions that are already covered in these documents. Also, it's a good idea to print out a copy of the course syllabus and schedule. Most everything the student needs to know about the class is in one of these two documents, i.e., where the points come from, how to earn them, when assignments are due, general info on the class (like the email system) and other vital information.
If you're having trouble meeting assignment due dates, have a personal problem that prevents study, or have ANY questions about the course (content, schedule, discussions, grading, etc.), let your instructor know. Be sure your requests are valid, though - and please don't say your dog ate your homework!
Don't wait until the last minute to take online exams, turn in assignments, etc. This is ALWAYS when computer or ISP problems seem to strike, so avoid potential last-minute hassles and delays!