In order to use categories, you must have the “Allow discussions to be categorized” checkbox checked on the Application Settings form. By default, there is just one category titled “General”.
There are really only 3 important things you need to do in order to create a new category.
Note: If you are running a Public Forum, you will want the Unauthenticated role checked on any categories you want available to non-registered users.
After you have at least 2 categories you can then drag and drop the categories to sort them in any order you wish. Your changes are saved automatically.
When deleting a category, you must choose a replacement category. Any discussions in the category you are deleting will be moved into the replacement category.