postgresSQL: howto start the machinery

[Olivier Billet]

Hi everyone,

i'd like to use postgreSQL but running createuser as root
throw me this error message : 

| $  createuser 
| Connection to database 'template1' failed.
| FATAL 1:  SetUserId: user 'root' is not in 'pg_shadow'
| 
| createuser: database access failed.

how to set up the postgresql machinery ?

Thanks,
Olivier.