To install MediaWiki 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.
Go to MediaWiki website and download the latest MediaWiki stable full version in .tar.gz format on your desktop and extract it. Now you will get mediawiki-1.16.2 folder. Here 1.16.2 is the version number of current stable release.Just rename that folder to mediawiki.
2.Copy mediawiki to /var/www folder