PostGIS has been installed on ubuntu server 11.10...
Now how do i create a database, admin user, etc..
where does it get installed
ls -l /etc/...?
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.