Cannot create database

First i want to say that i just subscribed to this mailing list and this
is my first post. :)

im new to postgresql and i installed it recently on linux redhat 9
system following the instructions from administrators tutorial.

then i attempted to create "my first database" following the
instructions from tutorial, but i get this message:

[root@localhost root]# createdb mydb
psql: could not connect to server: No such file or directory
        Is the server running locally and accepting
        connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
createdb: database creation failed

can someone help me? whats wrong? i am logged in as root, just started
learning postgresql...