When you are programming something new the docs came first than everything else.
According to the PostGIS description this software only adds a support to a particular type of object, in other terms and simplifying the concept, PostGIS is only a "plugin" for PostgreSQL. Considering this your question is about learning PostgreSQL and not really about PostGIS so i suggest to read the PostgreSQL documentation http://www.postgresql.org/docs/ or search on the internet for PSQL tutorial.
Best Answer
this package is not available in the official repository so you can use a ppa for a quick and dirty installation.
https://wiki.ubuntu.com/UbuntuGIS
for Ubuntu 11.10 Oneiric the packages are only available in the unstable version of this PPA, to install and use this ppa
to update your software list and install the package you are interested of