By default, you will need "sudo" (substitute user) privileges to install or change any system level programs or features due to security. This password will be the same as the password you had set when installing Ubuntu. Root is a different user with a default password, that's why when you do the "su" command and try and provide the root password, it never works. Okay the problem is that you can log on as a normal user, but not as root, because you do not know the root password.
- Reboot your machine, when you are at the reboot screen on the "Grub Boot Loader" menu screen, press the escape key which will present you with a menu.
- Pressing ESC in the previous step will bring you the menu.
- Highlight the line that begins with the word "kernel" and then press 'e' on your keyboard to edit it.
- Add the following at the end of the line that you are editing, exactly, without the quotes "init=/bin/bash" and hit enter.
- You will now be at the shell prompt logged on as root without specifying a password.
- Now you should change your root password and make it what you want it to be. This can be done by issuing the command "passwd" (without quotes). It will prompt you to enter a new password and confirm it again. It should be able to take a password as small as a single letter, once you set the single letter password. You should boot into Linux and change your single letter password to a more strong password for security.
- Reboot the machine, log on as a normal user, then do a sudo passwd root, and change the password for root by specifying the current password and then giving new password, followed by confirming it again.