Sections in MediaWiki are created by creating their headers. You can use the example below and replace Section, Subsection and Sub-Subsection with the names you would like to give to your sections:

==Section=====Subsection=======Sub-subsection====

Here is a preview of how this example will look after the parsing:

In order to add a page to a category in MediaWiki, you need to use the following syntax:

[[Category: Category Name]]

You can add a page to multiple categories.

The result will be:

To create the category itself, you can click on it and edit and save it just like any other page in MediaWiki.