sudo - My password can log me in, but nothing else ("user is not in the sudoers file" error when I try)

I recently got a new user account for Ubuntu 12.04 on Linux after becoming unable to log in to one of my earlier accounts. I can log in to this account easily enough, but when I try to use the password to install software through the Ubuntu Software Center, it reads:

Your authentication attempt was unsuccessful. Please try again.

I tried installing the software through the terminal with this:

sudo apt-get install

But when I entered my password, I got this message:

[username] is not in the sudoers file. This incident will be reported.

I searched here for a solution, and I found the answer here, which told me to open a root terminal. I don't know how to do this, but I found out I could get a root terminal by entering this in a normal terminal:

sudo -i

Upon doing this, I was asked to enter my sudo password, and I got the same message as before. What am I missing or doing wrong?