The Header Image
The header image is like the cover of a book, it gives readers an idea of what to expect when they visit. You can use a static image or a rotating series of images, as preferred. It's a good idea to place your blog name somewhere on the header image, as well. The header shouldn't take up the entire screen. A bit of the blog post below the header should show without scrolling.
Blogs typically have two or three columns. Generally, a two-column layout works fine unless you have a lot of diverse information to present or a lot of advertisers, otherwise the extra column just makes the blog busy and hard to navigate. Your content should always be the main draw on the page, not the columns.
“Above the fold” is a newspaper term borrowed by websites to refer to everything visible on the reader's screen before they begin scrolling. Your header and the beginning of your first blog post should sit above the fold. In the columns, generally the “about me” information followed by an advertiser block is visible. Below the fold is the place for archives, link lists and additional advertiser blocks.