PostgreSQL packages are also available in default Ubuntu repository, But when I try to install PostgreSQL on Ubuntu 12.04 I only found PostgreSQL 9.1 was in default apt repository. So I request you to add PostgreSQL apt repository first in your system suggested on official PostgreSQL website using following command.
$ sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ `lsb_release -cs`-pgdg main" >> /etc/apt/sources.list.d/pgdg.list'
$ wget -q https://www.postgresql.org/media/keys/ACCC4CF8.asc -O - | sudo apt-key add -