To install joomla 1.6 on your localhost, you must setup apache and mysql first. We have already posted a tutorial on How to install and configure Apache,PHP,MySql and phpMyadmin on ubuntu.Please read first.
1.Download Joomla 1.6
Go to joomla website and download the latest joomla stable full version in zip format on your desktop and extract it. Now you will get Joomla_1.6.0-Stable-Full_Package folder. Here 1.6.0 is the version number of current stable release.Just rename that folder to joomla.