My son's birthday is coming up and I would like to do a baseball theme cake. I have no idea how to cover a Styrofoam ball with fondant so I am hoping someone can help me out with this question. Thanks!

How big of a ball are you looking to have? If it's just a small decorative ball for on a bigger cake, I would say just make the whole thing fondant/gumpaste.

I've done this before. I just wet the styro ball before covering. Then I rolled out the fondant leaving it a little thick. Then I slowly worked the fondant over the ball, really taking my time until it went all the way around the ball and I just squeezed off the excess. It was a lot easier than I thought it would be. Here's a link to the cake where I did this in case you want to see an example:
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1009453