You installed Joomla and ready to add content to your website. If you will publish too much content with your website I suggest you to create sections and categories and publish your websites content under a proper section and category. If you website will contain just a few pages, you may not need to create sections and categories. I mean it is possible to publish your content as uncategorized.
You can create sections and categories from easy to use administration panel of Joomla. In case you wonder you did not do anything wrong, I share a sample video that shows how to create sections and categories in Joomla.