Blog Help

So I want to revamp my blog...hence the lack of a background right now. My friend did my banner for me (thanks Melissa!) but I can't find a new background that I like to match it. My problem is this: how do you create your own banner? I have a Mac, so any of you that do as well let me know what program you use. I am also having a hard time finding a background that is "me", that is 2 column not 3 (don't know how to do the 3 column thing). So if you have any advice, leave me a comment please!