Create New Category
Creating a new category is very easy. When you create a new article you need to add special wiki text at the end of the article. The format of the text "Category:categoryname" enclosed in two braces (where categoryname is the new cateogry name).
For example the category for this page is "Editing Tutorial". At the end of this page, I inserted the following wiki code:
After inserting this code, save the changes to your article, you need to click on the category link. That will bring you to the new category page. You can then edit it and add text that describes the category. Then save changes to the category page and you have created a new category.