Please do not create main categories... instead make subcategories (see testsubcat) and fit it into an existing main category. If you feel that a new main category is really needed, please discuss it at the Village Pump.
To start a new subcategory edit the page 'Category:your subcategory name' and then at the bottom of your wiki enter the line [[category:the category name that your subcategory is a part of]].
For instance: the line [[category:testcat]] should make this 'testsubcat' a subcategory of the category 'testcat'.