Header Image and Navigation Bar fixed when scrolling

Hi I was just wondering how to stretch the header image across the whole width of the screen, as well as the navigation bar. I also want to be able to scoll down and have the header image and nav. menu follow the page. I have tried playing around with the code somewhat, changing the position of the header image in the css editor but I am still somewhat of a noob at this. Please Help!
The site isn’t actually up yet as we are working on a temp site still, but this is an example of what we are trying to achieve:
http://ambercentre.ie/education/