Fixed header image

Hi there,

I’m trying to get my header image to be fixed. I have no CSS knowledge, so this is proving to be really frustrating. My website is https://www.jenniesteinberg.com. As you see, it is currently programmed so that the sidebar is fixed (which is good) but the header isn’t (which looks weird). When I set the header to “fixed,” it reformatted the whole page and put the sidebar over the header and the body of the website under the header, so that the first thing to appear was the middle of my face, and you couldn’t see the pictures on the side of the header. Anyone have any ideas how to fix this? Here’s the code from my stylesheet, if that helps:

[Please stop posting your entire 2,300 lines stylesheet. Post a link to your site instead.]