To use GeSHi in order to get your code highlighted in Joomla, you first need to enable the GeSHi plugin via the plugin manager.
If GeSHi Highlighter did not come pre installed you can download the GeSHi package from the developers site. Copy the php files from the "geshi/geshi" directory in the package to the /libraries/geshi/geshi/ directory on your server.
Below is the code you will need to use. It is encapsulated within the code itself to show how it looks when used. Insert the code below in the html editor within a Joomla! article. Change "YOUR CODE HERE" to whatever code you want embedded in the article. Remember, this is t show code, not execute code.
<pre xml:lang="html"> YOUR CODE HERE </pre>