To add a new category to a page you can click the Category button at the bottom of the page and type in the name you'd like your category to be called. Though curiously, this is outdated and no longer working.
In source mode, type in [[Category: <insert category here>]]. In visual mode, the category sorting is available at the sides.
A full list of existing categories can be found at Special:Categories.
This category has the following 7 subcategories, out of 7 total.