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. Please check out the full list of existing Categories before creating a new Category to see if the new Category is definitely necessary.
This category has the following 6 subcategories, out of 6 total.
Pages in category "Browse"
This category contains only the following page.