Before Starting to work with PostgreSQL

Who starts to work with PostgreSQL on Linux has a bad initial experience: to do the first authentication. The installation process of this database server creates a SO user, the "postgres", which has super user access, able to do all initial steps of a beginner programmer, like to create users and databases. But, how can we use this user if we don't know its password?
I followed the steps below, which was compiled from different Google's sources:

1. change to the root user or a sudo user, who is able to change to another user without informing its password:
   # su root
   or jump step 2 doing:
   # sudo su postgres
2. change to the Postgres user:
   # su postgres
3. create a database server user with the same username of yours:
   # createuser -d -r -s -P (your_user_name)
   This command will create a database server super user, able to create new databases and roles. The command will request a password and then the user will be created.
4. change to your user:
   # su (your_user_name)
5. execute the command to create a database:
   # createdb (database-name)