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.
A full list of existing categories can be found at Special:Categories.
This category has the following 3 subcategories, out of 3 total.
Pages in category "Browse"
The following 27 pages are in this category, out of 27 total.
Media in category "Browse"
This category contains only the following file.