When I was starting with Ubuntu I was struggling with getting QGIS to run on Ubuntu. Actually it is not a big deal despite of adding a repository to your system. So in this video I am starting with a clean install of Ubuntu 12.04 Precise Pangolin so everyone can follow me:
the lines to edit in your sources.list file are:
deb http://qgis.org/debian precise main deb-src http://qgis.org/debian precise main