On a standard desktop installation, I created my administrative and user login. Further with some applications, the system ask for the root password.
So my question is: What will be the default password for the root login when I never created it during the installation process?
By default root does not have a password and the root account is locked until you give it a password.
When you installed Ubuntu you were asked to create a user with a password. If you gave this user a password as requested then this is the password you need. If not, or you have forgotten it, then you need to set a password.
Instructions on how to do it are here: How do I reset a lost administrative password?
Once you have an administration password you can use sudo to run commands that would otherwise require you to be root and enter your password.
If you want to set a root password see this question How to enable root login?
However, read all the answers carefully first to make sure you want to do this. It's not recommended and many of the answers explain why.
Open terminal and type following command:
sudo passwd root
then press enter button. Type in your new 'root password', like '123456' and verify the new password, by entering it again, followed by the enter button.
Now your new 'root password' is set to '123456'.
Are you sure that the system is asking for the root password?. It usually asks for your password to request elevation.
