If you feel that certain content should only be visible to registered members of your forum there are several methods to restrict what unregistered guests can view.
Guests Must Login
To prevent guests from viewing your entire forum until they register, visit your admin area and select Settings. On the following page, select Forum Settings. Scroll down to the Login Settings heading and find an option titled Guests Must Login. Change this setting to Yes to prevent guests from viewing any content on your forum. You'll also have the option to enter a message to display to any guests who attempt to visit your forum's URL.
Hiding Categories from Guests
In your admin area, select Structure, followed by Categories & Boards. Hover over any of your existing categories, then click on the icon next to the category you want to edit. In the window that pops up, change the dropdown next to Who can access this category... to your preferred choice of access restriction for that particular category.
Hiding Boards from Guests
In your admin area, select Structure, followed by Categories & Boards. Hover over any of your existing boards and click on the icon when it appears to the right of the board name. Click on the Permissions tab in the window that pops up. Change the dropdown next to Who can view the board... to your preferred setting for that particular board.
Several other sections of your forum will also have either an Edit button or a dropdown that will grant you access to the same type of permission settings for their respective content.