Setting up a public forum is pretty straightforward.
- Create a page for the forum (if there is not one already). The page style needs to be "Forum".
- If you want it available somewhere in the site navigation, be sure to set the parent page.
That is all you need to do for any forums that are already created. They should now show up on this page. If no forums have yet been created, or if you want to add some or change what's there, carry on...