How do I create a central message board where my users can post news?

You can decide for each category whether it should serve as a message board or not. A category serving as message board can easily be accessed by all your members. When accessing their text editor to write a new entry, any message boards will appear in your members' category list. Thus, they merely need to choose the message board instead of one of their own categories, and any new posting will appear in that central message board. In order to distinguish between their own categories and your message board(s), the latter are displayed at the end of the list, separated by a dashed line. You find your categories listed at "Portal content->Categories".