Can't switch the postgres user..

8. Initialize PostgreSQL (*when running under the "postgres" account*):

mgonzalez@guaj215desa ~
$ initdb -D /usr/share/postgresql/data
The files belonging to this database system will be owned by user "mgonzalez".
This user must also own the server process.

The database cluster will be initialized with locale C.

Fixing permissions on existing directory /usr/share/postgresql/data... ok
creating directory /usr/share/postgresql/data/base... mkdir: cannot create direc
tory `/usr/share/postgresql/data/base': Permission denied

initdb failed.

mgonzalez@guaj215desa ~
$ su postgres
Password:
su: incorrect password

As you see, I tried to change the current user to postgres.  I left the password blank by pressin ENTER twice.

What I am doing wrong (again).

Thank you in advanced for your patience.