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First you need to install PostgreSQL. PostGIS can be installed via the "Application Stack Builder that was installed with PostgreSQL".
After launching this \installer you will pass the following steps:
PostgreSQL/PostGIS installation on Ubuntu
A description on how to install PostgreSQL and PostGIS on Ubuntu can be found here:
http://trac.osgeo.org/postgis/wiki/UsersWikiPostGIS21UbuntuPGSQL93Apt