Decide where to install the PostgreSQL database. Most of the times installing the on the database on the same machine is ok.
Install PostgreSQL and PostGIS
sudo apt-get install postgresql postgresql-9.3-postgis-2.1
Optionally install also PGAdmin
sudo apt-get install pgadmin3
Change the postgreSQL password
sudo -u postgres psql -c "alter user postgres password 'postgres';"
Create your PostGIS database
For Postgresql 9.1 and later versions:
sudo -u postgres createdb -E UTF8 istsos
sudo -u postgres psql -d istsos -c 'CREATE EXTENSION postgis'
Note
For older versions of postgresql / postgis:
sudo -u postgres createdb -E UTF8 istsos
sudo -u postgres psql -d istsos -f /usr/share/postgresql/9.x/contrib/postgis-1.5/postgis.sql
sudo -u postgres psql -d istsos -f /usr/share/postgresql/9.x/contrib/postgis-1.5/spatial_ref_sys.sql