Many courses use discussion forums to provide students with a way to communicate with their peers and instructors. You can access the forums by clicking the Discussion tab inside the course.
Once there, you have several options available to you. You can browse or search existing posts (#1 below), or – if nobody else is already discussing your topic – you can add a new thread with the "Add a Post" button (#2). When making a new post, choose whether it is a question or discussion as well as where it should appear (#3), then write the title and body of your post (#4).
(If your post is a question for the course staff, include the word [STAFF] in brackets in the title to get their attention. Posts made by course staff will have a blue "STAFF" marking on them, as on the left side of the screenshot above.)
You can interact with existing posts in a few ways. Click on a post title from the list on the left of the page to view its thread. You can post a comment if you have something to contribute to the discussion. Use the menu on the top right of the page to upvote the post (marking it as valuable), follow the post, or report it as inappropriate (see below). You can also edit or delete your own posts with this menu.
Remember that everyone is here to learn. Be respectful and generous toward other forum users. And don't share answers to assignments unless the instructor has said this is ok!
Some courses may not use the discussion forum at all. Others may use outside forums, like Piazza. If this is the case, the instructor will explain how to access and use the forums in a course announcement.