Header?

I finally got my blog working again, but my header wasn’t there… so when I tried to put my header on… I ended up screwing it all up and I was wondering where I could find a clean copy of the wordpress default theme’s header.php… and how, once I have that, do I put my header in it? (http://www.dharmabakery.com/blog/wp-content/uploads//header_3.jpg)