You can take a look at this link click here and if you right click the Source code hyperlink and save the text file you'll see how it all goes together.
There are one or two other ways of getting a page element to move as the page scrolls, but doing so is tricky at best and much depends on how your navigation menus have been set up.
Without further information from you, it's the best I can come up with on short notice, but you can keep in mind that there are alternatives to the issue.
Post again with more detail if you can and one of us will walk you though something.
I'm offline a lot at the moment so I don't get into the Helproom or WebDesign forums as much as usual, but if you need to you can email me (click the envelope icon next to my name) and I'll get back to you when I can.