This video tutorial from ChaminadeENN shows how to set your Facebook account to private mode.
First what you need to do is to sign in to your Facebook account.
When Facebook account main window appears, click Settings button from top menu.
Find Privacy option and click Manage button. When privacy settings window opens click Profile option.
In the newly opened window you will be able to change settings and access level for all your Facebook activity and profile information.
From Basic tab, set all settings to Only Friends from drop-down menus. When you've set everything, click Save Changes.
Now your Facebook profile is in private mode. Voila!