Before you jump into making your own articles, categories, tags, web links, contacts, banners, and news feeds, you may want to review Planning Your Web site. There are many aspects of creating a web site to consider. For example, deciding if you want to enable Search Engine Friendly URLs and how enabling or disabling Search Engine Friendly URLs later will destroy the rankings your pages acquired with search engines such as Google, MSN and Yahoo.
Perhaps you already have a website. Then you should read "Converting an existing website to a Joomla! website" and learn more about converting your static website.
One of the first tasks you may want to do is to modify one of the templates that comes with Joomla! so that it displays your site name and logo.
Finally, you may want to install one or more of the thousands of extensions to Joomla!. Most Joomla! extensions are listed in the official Joomla! Extensions Directory (JED). Once you've found an extension to install, Installing an extension will tell you how to install it.
Please remember that all extensions are developed by independent developers. If you need help with a specific extension the best source will be that developer.
The Joomla! forums are the among the busiest (and most friendly) support forums in the world, and they are a great place to get help and meet other Joomla! users.
Here are some quick links to specific Joomla! forums commonly used. General/New to Joomla! 3.x Questions, Joomla! 3.x Template Questions, Joomla! 3.x Extension Questions, General/New to Joomla! 2.5 Questions, Joomla! 2.5 Template Questions, and Joomla! 2.5 Extension Questions