Best thing is to hire someone if you are not a designer.
But now Twitter backgrounds are OUT. Yes they are- as MOST people visit Twitter from Apps like Tweetdeck, where they cannot see the background.
If you really want one, check out chris spooner's blog for a tutorial.
EDIT: Here is the link- http://www.blog.spoongraphics.co.uk/...best-practices
EDIT AGAIN: Just wanted to add, my background, got featured on the Top 10 backgrounds of Bloggers. David Risely and others were in it :O haha!