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 builidng your own site.
The first site is a simple site about Australian Parks. It shows you you an 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 off 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.
As you make your Joomla! site using the extension you will control the details of the display using parameters. Parameters control everything from whether the author's name is displayed to who can view what to the number of items shown on a list.
Default parameters for each components are changed using the Options button on the component toolbar.
Parameters can also be set on an individual item, such as an article or contact and in menu links.
If you are happy with how your site looks, it is fine to leave all of the parameters set to the defaults that were created when your site was installed. As you become more experienced with Joomla! you will use parameters more.