Locked out of postgres on my redhat 7.3

I'm totally new at postgres, but I wanted to try sql-ledger and it requires
it so I thought it seemed like a great opportunity to dive in.
Unfortunately within a matter of a few minutes I've somehow managed to lock
myself out completely. I tried to use the Webmin .99 server and it seems to
let me in, but I cannot add users, or do anything without getting permission
denied. I get the same errors when I su to postgres and try to createuser.
# su postgres
bash-2.05a$ createuser -d testuser
Shall the new user be allowed to create more new users? (y/n) n
ERROR: CREATE USER: permission denied
createuser: creation of user "testuser" failed
bash-2.05a$

I tried uninstalling and reinstalling but no matter what I do I can't seem
to get in. Feeling dumb, and getting no closer to success. Any help would
be very welcome :)

Robin