code in child theme

How do I change the stylesheet of a twentyeleven child? I am new at this, and I have read all the instructions several times as well as searching the forums. Do I copy all the code from the parent stylesheet to the child? Or if I make changes in the parent, do those changes affect the child but remain permanent if the parent is updated? To be more specific, I am trying to make the central header, graphic, menu, etc. in twentyeleven to automatically resize to the size of a screen instead of being smaller in the middle of the screen. Thank you for your help.