Permission Problems:-)?

I'm trying to build a database.  But none of the users I try to use works.
I as normal user doesn't work:

:~> createdb mydb
createdb: could not connect to database template1: FATAL:  user "adriel" does
not exist

I as postgresql user doesn't work:
:~> createdb mydb
createdb: could not connect to database template1: FATAL:  user "mypostgre"
does not exist

I as root user doesn't work:

# createdb mydb
createdb: could not connect to database template1: FATAL:  user "root" does
not exist

I can only start postgresql as root.  What am I doing wrong here?  How do I do
it right?

Thanks,
Jerome