Here's a high-level overview of Joomla's organizational structure:
The main body of any page is referred to as the content. At the deepest level, this would be a single article of content. Articles are organized within categories and each category is within a section. The content of a page can be set to a single category or section, showing teasers for all the articles within that container.
Elsewhere on the page can be little snippets of data called modules these encompass all other text that is not the main content. This includes footer text, a rss feed display, and the display of menu items.
Once content (articles) is created and organized on the administrative side, the navigation structure needs to be designed. Joomla has menu containers that contain menu items. Once you have a menu container created, you must create a module to tell Joomla where on the page to put it (most templates have various zones to place modules into, around the main content), so it will be displayed.
So, create a section, create a category in the section, and create an article in the category. Create a menu container, create a menu item in that container that points to the section. Create a module to show the menu container. Then you should have a basic site that has one menu option, that when clicked shows the categories in that section, and if you click the one category listed, will show the one article in that category, and when that article title is clicked, will show the article.