Contrary to what their most common use would lead you to think, `su` and `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 `sudo su`.