Go to the QEMC in version 10 or QMC in version 11, browse to Documents, User Documents, expand your server and folders, and click on the document you want to categorize. On the right part, click on the tab "Document Information". Select one value from the drop down list, or type a new one. Once you have typed a new category and clicked on Apply, this category will be available to any other document.
Hope that helps.