Use Fink and FinkCommander to install PostGreSQL (the binary package or from source). The postgres user will be created as well as the databases' folder /sw/var/postgresql-8.0 (I installed the 8.0.6-1026 version).
Start the PostGreSQL server (interactively)
sudo -u postgres /sw/bin/postmaster -D /sw/var/postgresql-8.0/data
In another terminal, connect to the template1 database (built-in PostGreSQL database)
psql -U postgres template1
Create a super user for you Mac OS X account (very convenient). My Mac OS X user name is jerome.
CREATE user jerome createdb createuser;
Create another user with a password (for your web application ?)
CREATE user user01 createdb; ALTER user user01 WITH password 'pass01';
Log out
\q
Create your database as your new user
createdb -U user01 test01
To stop the PostGreSQL server, just type Ctrl-C in the first terminal.
That's it ! Start your real work now !