[Theme: Twenty Twelve] How to add a gap between the nav bar and the body?

Hello. I am using the standard Twenty Twelve theme, and I have created a child theme already. What I want to do is to un-stick the navigation bar and the body from one another and to add a gap between them so the page would look like this: http://grab.by/mUss

For a beginning, I can say that I absolutely have no clue how this could be done.

Thanks in advance!