You can sign in as ROOT in Ubuntu Terminal without password in the first time by typing
sudo bash
After that you can change the password for the ROOT by typing
sudo passwd root
And then you type the password. Don’t afraid when you type there is no character appears because it is the default. It is recording what you type.
If you’ve changed the password you can sign as ROOT by typing
su root
or
su
For further reading you can read Sudo Manual and SU Command in Wikipedia