Adding and Managing Categories
From Georgetown University Commons Knowledge Base
To add a category, click the drop down arrow on the "Post" button in the left sidebar then click on the "Categories".
Type in the category title in the title box then click the "Add Category" button. If you wish to create a sub-category, you can click on the "Parent" drop down menu and select another category you wish to mark as a parent to the sub-category you are creating. The parent will act as the larger category under which your sub-category will appear. The sub-category should appear slightly indented underneath the main categories when viewed on the post editing page.
Once you have made a category, it will always come up as an option to select when creating a new post. You can assign a post multiple categories, or you can opt to leave it uncategorized.