This is actually the standard way in which the forums will work, of you use the permission settings as they are "out of the box". The Public will be able to read the posts, but only logged in site member can add content.

You manage this using the Permissions settings for the Discussion Forums. There are some general instructions on these here.

http://cambridgeopensystems.com/documentation/community/administering/permissions/

What you're looking for is Read access for The Public, but only allow Contribute access for Site Members.

Thanks

Joe - Voice Admin