As far as I know, you can't make the background of a site animated, unless you use flash to create the site.
In any case, this would require knowledge of Photoshop, Flash or other animating programs of the sort.
I'm not sure if you can accomplish this with a script.
Either way, it seems like you're looking to do something that is beyond your capabilities at the time. I would look into how to do this, and start trying to further your skills with the required applications (Flash, for example). There's a TON of free tutorials you can find online, for any application or [programming] language.