First of all you need to have admin privileges. If you don't have admin privileges then while booting press and hold the shift key. You will enter the GRUB menu. Here select recovery mode. When you see the list of options like
dpkg etc, choose to start a root shell and type:
adduser username sudo
username is your username :-) )
this will add you to the sudoers group! Then type
exit and boot normally. when you enter your account, type:
sudo passwd root
Then enter your password and type the new root password
After that you can type
su and enter the 'root' password.
If you don't want to change the root password then you can use:
to start a root shell, using your own password.