Next step in organizing the content in your website is creating the categories. To do that you need to go to the Category manager through the Content menu:
There you will be able to see the existing categories, edit and manage those and create new ones. Click on the New icon to start the creation process and fill in the details required for the new category.
It is important to select the parent Section for the new category, in order to have the proper content organization (or at least the one that you desire).
Again, you can set the menu linking details after saving the category.
Please note that an empty Section or an empty Category will not show up in the menu. Each of these must have at least a single Content item created and published.