In forum management make a new forum.
Forum type: Category
Parent forum: No parent
Set permissions for category.
Then make another forum.
Forum type: Forum
Parent forum: the category you just made.
Set permissions for forum
To set permissions, either use the option to copy permissions from an existing category or forum that has the desired permissions, or:
Administration Control Panel (ACP) > Permissions > Group forum permissions
Set the registered users to be able to see and post in it:
Registered users > Standard access
"The more connected we get the more alone we become" - Kyle Broflovski©