To login to the front end of your site use the login form or the login menu link on the "This Site" menu. Use the user name an password that were created as part of the installation process. Once logged in you will be able to create and edit articles.
In managing your site, you will be able to create content that only logged in users are able to see.
Once you are logged in, a new menu will be visible. To create a new article, click on the "submit article" link on that menu.
The new article interface gives you a lot of options, but all you need to do is add a title an put something in the content area. To make it easy to find, set the state to published an put it in the category.