the easiest way is to go to one of these sites that create the background for you.....
google it
one is pimpmypage.com
if you see someones page that you like usually there is a bit that says something like "my page was created by xxxxxx" and its a clickable link
hope that helps somewhat!
look on the net thers loads of Myspace profile sites its easy just copy and paste the selected HTML for the back ground you want into your myspace profile.
They are the only one I could find with no pop-ups and they have really unique designs for myspace. I'd just go there and if you just want a plain background go to their plain background section and you can get one.