Follow the following steps to make a category

Content > Category Manager > Add New Category

or you can first go to

Content > Category Manager

...and then click on "NEW"

Give it a name say "Technology" and press "Save and Close"

or else you can choose a "Parent" if you are making a subcategory. For example for making a subcategory, "SketchUp" of Category "Technology", I would choose "Technology" as "Parent"

 Now you are ready to make an article under this category. See the tutorial on article.