By default, when you install Ubuntu , there is no root user password. You have to manually set one before using root account .You will need to use your own account password (password used while installing Ubuntu) in order to assign password for Root user. Here is how you will do this(using sudo passwd command):

packetwisdom@Ubuntu:~$ sudo passwd

[sudo] password for packetwisdom:

Enter new UNIX password:

Retype new UNIX password:

passwd: password updated successfully

packetwisdom@Ubuntu:~$ su root

Password:

Added user root.

root@Ubuntu:/home/packetwisdom# exit

packetwisdom@Ubuntu:~$

When you log into as root user , prompt will change from dollar sign  ($) to pound sign (#) .You can switch back to your account using ‘su username‘ (switch user command).