Question on Starting Postgres for the first time

I am a new user of PostgreSQL and Linux.  I have my Linux up and running and
my PostgreSQl semi-configured.  I can get into PostgreSQL using super user
on root and then super user into postgres.  The problem is when I try to run
psql and createdb from my home directory.  When I do that, I get the
following messages

$ createdb david
psql: FATAL 1:  SetUserId: user 'david' is not in 'pg_shadow'

$ psql
psql: FATAL 1:  Database "david" does not exist in the system catalog.

Not sure what is going on here, but I think someone out there does.  Can you
help.

Thanks
Dave