hat password do I use to download items on my pc

I'm not sure I'm in the right place. I was told to go on ubuntu software center to be able to download more games. However, it asks for a password and the ones I try to put in it says is wrong. I'm using the password I was given with the computer, then I use the one I reset it to when I setup my computer. What password authentication is it looking for? Any help would be greatful.