HTML/CSS Snafu

I've made a new layout but I'm having issues getting the html/css to work the way I want it to. I want my header to be fixed so that the blog content scrolls under the header and the header stays at the top of the page. I found a code (http://fixed-header.blogspot.com/) to do that and it works except that my blog's content starts halfway behind the header which is not okay.

I'm also having a problem where the background image is for some reason showing up enormous when it is supposed to be the same width as the header.

I've searched for answers to my problems, but all I'm finding is a whole lot stuff from 2008 that doesn't seem to work. Maybe because I'm using chrome? I don't know :-(

Can anyone nudge me in the right direction?

At this point I just need the background image to stop stretching to ginormous proportions.