I need to know password for postgres

I need to know the password for User postgres.  I am trying to set up
gphotocoll which uses pgsql.

The setup insructions are

1. login as postgres - which I can only do by su  and then su postgres.
2. createdb - no problem
3. launch gphotocoll as postgres.  I have to supply the password for
postgres and I don't know it.

I'm running pgsql 7.2.1 RPMs on Redhat 7.3.  Postmaster is running.

If you can help, please don't assume too much knowledge on my part of
Linux! I've still much to learn.

Thanks

Dave