can't log in as 'su'

I am trying to install java and the instructions to install a rpm file is type su and the password.....ian@ian-desktop:~$ su
Password:
su: Authentication failure
Sorry.
ian@ian-desktop:~$ sudo
usage: sudo -K | -L | -V | -h | -k | -l | -v
usage: sudo [-HPSb] [-p prompt] [-u username|#uid]
            { -e file [...] | -i | -s | <command> }
ian@ian-desktop:~$
What now? The password is correct.