Contrary to what their most common use would lead you to think,
sudo are not just meant for logging in (or performing actions) as root.
su allows you to switch your identity with that of someone else. For this reason, when you type
su, the system needs to verify that you have the credentials for the target user you're trying to change into.
sudo is a bit different. Using
sudo allows you to run certain (or all, depending on the configuration) commands as someone else. Your own identity is used to determine what types of commands
sudo will run for you under someone else's identity: if you're a trusted user (in the sense that the sysadmin trusts you), you'll be allowed more free rein than, say, an intern. This is why
sudo needs to verify your own identity rather than that of the target user.
In other words, trying to
su to someone you're not is like attempting to charge your purchases to a stolen credit card while using
sudo is like selling your friend's car by legal proxy.
As for what you were trying to do, just
sudo su root, or even more simply
sudo su and type your regular user password. This would roughly amount to replacing your friend's credit card credentials with your own using the legal proxy they gave you :). It of course assumes the
sudo configuration allows you to run
su with escalated privileges.
Also, systems that come pre-configured with
sudo access typically have the root account disabled (no root password), you can enable that using the
passwd command after becoming root via