Joomla is a content management system. The words and images that make up your site are stored in a database, a template controls how they look, and Joomla assembles the two into the pages a visitor sees. Because the two are kept apart, changing the template changes the look of the whole site without touching a single article.
Two sides to every site
The front end is the site your visitors see. The administrator, or back end, is where the site is managed. Reach it by adding /administrator to the end of your domain name and signing in with the account created when Joomla was installed.
Adding content
In the administrator, go to Content → Articles and choose New. Give the article a title, add the text, pick a category, and save it as published. An article on its own is not yet part of the navigation — to put it in a menu, go to Menus, create a menu item, and point it at the article.
Learn more
There is far more to Joomla than one page can cover. The Joomla user manual, the documentation site and the Joomla forums go into detail on categories, user permissions, multilingual sites and everything else.