Categories are Wikipedia way of tagging entries to connect individuals who share some common trait.
Generally use the most specific subcategory, for some categories people should be in both more general and more specific, especially subcategories that are broken out by gender or sexuality. This prevents for example: all American women writers being removed from the American Writers category.
Example: LGBT Sportsperson
Once you've selected an article you can add appropriate categories by editing it in the visual editor, expanding the hamburger menu and selecting categories.