Did I set up my Ubuntu system incorrectly?

I have an Ubuntu system stored on a Virtual Player. I am studying command line administration for linux and am using ubuntu to do this. I tried to create a new user account a few minutes ago.

I used the following commands

su -

it asked me to enter a password, when I entered the password for the only account on the network. I got an error message that said Authentication failed.

Earlier, I installed a new program onto the system using the apt-get command. I entered the password I used to log on to the system and did not get any errors.

Can you tell me what I am doing wrong and how to correct it? Do I have to re-install the program all over again?

Thank You.