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 you want to use syntax highlighting for any language that didn't come pre-installed with Joomla, you'll need to download the GeSHi package from the developers site. Simply copy the php files from the "geshi/geshi" directory in the package to the "[JoomlaBase]/libraries/geshi/geshi/" directory on your server.
In order to highlight code, use the following syntax:
<pre xml:lang="html"> </pre>
