Your installation includes sample data, designed to show you some of the options you have for building your website. In addition to information about Joomla! there are two sample "sites within a site" designed to help you get started with building your own site.
The first site is a simple site about Australian Parks. It shows how you can quickly and easily build a personal site with just the building blocks that are part of Joomla. It includes a personal blog, weblinks, and a very simple image gallery.
The second site is slightly more complex and represents what you might do if you are building a site for a small business, in this case a Fruit Shop.
In building either style site, or something completely different, you will probably want to add extensions and either create or purchase your own template. Many Joomla users start by modifying the templates that come with the core distribution so that they include special images and other design elements that relate to their site's focus.