Adding a a left hand side bar to a theme

Hi there

i am wanting to add a left hand sidebar to the theme i am going to be using. I want to be able to add widgets to this sidebar. I do not have a clue where to begin doing this or anything. (i would be using a child theme for twenty twelve)

Im a complete noob at this kind of thing.

If it was possible could you please list the code to get it to work and where i need to add this code and what files i would need to copy over