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 11 subcategories, out of 11 total.
Pages in category "Browse"
This category contains only the following page.
Media in category "Browse"
This category contains only the following file.