Categories are the organizational header you see on the side bar in blue text. You'll want to create one of these before you add any pages for aesthetic and parenting reasons, but you can make as many as you think you'll need.
As an example, we chose to use it as a title and simply called ours "Wiki"
To create a category you need to first go into the Admin Menu. You can do that by going to the top right of any page where your name is, and then selecting the admin button.
Once you're in the admin menu, you'll have some options at the top of the dashboard. Go to the CATEGIRUES section, then ADD CATEGORIES subsection
The category creation process is extremely simple with only two real options you need to fill in.
1) Title: The name of the category as it will appear in the sidebar. The sidebar will automatically capitalize all the characters in the category.
2) Wiki: This is the wiki you want to add the category to. By default, it should select your currently viewed wiki. You will be able to see all the wikis you have the user permissions for as well, incase you want to put it somewhere else.
3) Save: When you're all done, just click save and it'll auto add it to the main page! You'll see a confirmation at the top of the screen. If you need to edit anything afterwards, you can do it from the Manage Categories menu.
That's it! Super easy right? Now that you have a category, it's time to add pages so you can start populating your wiki. Head over to the Create a Page article for the next step.