When installing ubuntu you were prompted to give a username and password.
This is what ubuntu is looking for. The password is the same as you would
use to install software. If that is not working something else is wrong. I
have what is the error message you are receiving?

It is possible to change this from a live cd... but more difficult.

Also, if you can still log into the system, make sure you have at least one
user with admin privileges in addition to the one you are trying to change.
The this is just so there is a failsafe.