Centuries ago, Nostradamus foresaw when "Habib Seifzadeh" <habib@sabaa.com> would write:

Try "psql -U postgres"; that should work better, although possibly
only a bit; it'll then complain that there is no database called
"postgres."

Try, instead, "psql -U postgres -l", which will list the databases
that exist.  (Probably just the "template" ones, template0 and
template1.)

One of the first things you should do is to create a user for your
_own_ id; there's a script called "createuser" that does that.  Once
you do that, you won't forcibly need to specify "-U".

The second thing to do is to, as that user, create a database that
you'll use to actually put data in.  "createdb" is, not surprisingly,
the appropriate command for that.

See the documentation at http://www.postgresql.org/docs/ for more
information...