Superuser su password reset
OS X doesn't set a root password by default, so it's not possible to log in or su to root. The preferred way to run commands as root is sudo, since that requires your admin password instead of the (nonexistent) root password.

sudo works a little different from su, as it's really intended to run single commands as root. Generally, you just use sudo as a prefix on whatever command you want to run as root (e.g. sudo cp -R myApplication.app /Applications). If you want a root shell (as su does by default), use sudo -s.

I don't recommend setting a root password, but if you do need to do it, you can either run sudo passwd or run Directory Utility.app (which is hidden in /System/Library/CoreServices/Applications), clicking the padlock and authenticating as an admin, then choosing Edit menu > Enable Root User.