Can anyone help me with a Javascipt ?

Ok i have no javascript knowledge whatsoever. But my client wants a java script alert window and a way to re-direct the page.

For instance user hits the back button OR clicks the X, or types in a new web address. Alert window shows. if the user clicks “okay/yes” – (yes leave site) – then let them go to whatever site they were trying to go to.if cancel (no stay on site) then redirect to a new html page on our site.

Any idea how I might do this. Or a good article that could help me figure this out by tonight.