Unable to create a new database because the "postgres" user isn't the database administrator

Hi everyone,

   it's the first time I set up a database manager at all.  I installed
PostgreSQL v7.0.2 on a RedHat 6.2 computer (Intel platform) using the
binary RPMs that I downloaded from the ftp.postgresql.org FTP site.

   I installed the following RPMs (PostgreSQL was not initially installed
on the computer) :

postgres-7.0.2-2
postgres-devel-7.0.2-2
postgres-perl-7.0.2-2
postgres-python-7.0.2-2
postgres-server-7.0.2-2
postgres-test-7.0.2-2


Note:   The "postgres" user was created during the installation of the
RPMs.


   The "postmaster" process can be started successfully.

   My problem is that the "postgres" user isn't registered as the database
superuser.  When I tried to create a new database with the command
"createdb myfirstdb" (logged on as the "postgres" user), I received the
following message:

pgsql: FATAL 1: SetUserId: user 'postgres' isn't in 'pg_shadow'


    I would like to know how to make the "postgres" user a database
superuser.


Thank you for any clues you may provide me.


Stephane Ouellette