Make sure that editing is turned on by clicking the Edit mode toggle in the upper right corner of your course page.  Scroll to the desired Topic or Week, and at the bottom of that section click Add an activity or resource. In the window that appears, select Forum from the list of options.

1. Give your forum a name in the Forum name box.  This will be what appears on your course page, so make sure it's descriptive.
2. In the Description box, write the question that you want the students to discuss in this forum.  If you want to discuss several topics, add a separate discussion forum for each topic.
3. In the Forum type menu, choose "A single simple discussion."
4. Scroll to the bottom of the screen and click Save and return to course.