starting the databaseserver

Hi there,
i have a realy silly question: I installed postgressql (v 7.2) from suse linux
8.0 and then i wanted to try the first steps with it.

As i understood, i shoud login as user postgres an then start the server by
some commands, wich are described in the documents.

It seems, the user postgres (group daemons) has been added by the
installation-procedure -at least not by me. Nevertheless, when i try to login
to this user, it fails because i do not have a password! So i can not take
the very fist step!

So my question simply is: How do I login to the user postgres?

Please excuse my bad english. It is not my native language.
Thank you for any information
Wolfgang