Well, start by doing a search herein for “default theme header change” (without the double quotes). Then after you look through the first 20 of over 1000 nearly identical threads, hop on over to http://themes.wordpress.net, find a theme which is laid out like you like, download, install, then start changing colors, graphics etc.
Honest. Any given theme from that link is far far easier to customize than the “default”. And don’t please be put off by black thumbnail boxes on the first few pages – Thomas is doing the site by himself, and gets a bit behind sometimes. Click on the black boxes, you’ll see the theme…. and “real thumbnails” start about page 4 or 5.