This is one of the core basics of Joomla and should be one of the first things that you learn when managing your Joomla website.
Adding content is a part of the day to day operations and processes of any website.
Adding a Content Article to Joomla
To add a new article to Joomla, log into the backend admin area of the website.
Navigate to Content->Add New Article
From here you can start adding in a title, choosing a category and adding in the copy for your website.
Click Save and Close when you are done.
It's as simple as that!