Many people come to Joomla! in order to use a site that is already up and running. In this case, you will need to know something about the site and where the Articles are that you are going to work with. This will vary a great deal from site to site, so is not covered here (obviously!) But there is background information about usernames and how to login.
Joomla! allows content to be created and edited by visitors who have a username (sometimes called a login) with appropriate permissions. The usernames are created, and the permissions defined, by the person who looks after the web site, usually called the Administrator.
If you are using an existing site, you will need to make sure that you have been given a username with the right permissions for what you are going to do. So you will need a username in order to be able to add and edit articles, and thus be able to use this hands-on Manual.
To login to a site - find the LOGIN area (this is often to the left of the page - but can be elsewhere). There will be a box for the username and one for the password.
You are likely to be able to see more menus on the site that you could as an ordinary visitor. And you should also be able to edit some of the content.