You're close. You need to read up on administrative stuff. specifically, root shouldn't be running ANY databases.
su - to the postgres super user (usually postgres) and issue the command:
createuser youusernamehere
answer yes to the create db question, no to create users (that power, and the super user power should really be restrcited to the postgres superuser and maintenance accounts only.)
Then, as yourusernamehere you can run 'createdb' and the dbms should then create a database under your name. 'createdb somename' will create a db called somename.
