By now you have no doubt become familiar with the HTML editor that appears in the Article editing window. For many people this is all you need but if you want more flexibility and features, you really should check out the alternatives on the Joomla Extension Directory. The most popular by far is JCE and it tends to be the first extension I install with any new site. It is easier to use, makes linking within your site a breeze and it even has a spell checker.
Check out JCE at www.joomlacontenteditor.net