The Bulletin Board is for stuff about your courses that doesn't change often.
By default you'll have three Content Channels enabled (the little box types)
Feel free to add others. None of the boxes are visible to students unless you have content in them.
To add things:
1) Select one of your classes.
2) Click "Enter Edit Mode"
3) By default, you'll see the three Content boxes (Announcements, Syllabus, and Links)
4) Click the Add Button on any of the content boxes to add that type of thing.
5) You can cross post anything you upload to multiple courses.