Its a standard single valve install. Most people to cover the holes use a remodel plate

Here is a link that might be useful: remodel plate

Fine Homebuilding Magazine did a feature article on this very topic a couple of years ago or so. The article is still accessible online. See the link below. You may have to sign up for free trial membership, but the article is quite good with very good step-by-step illustrations.

Here is a link that might be useful: Fine Homebuilding - Updating a Shower Valve

Stop by your local Home Depot book section and checkout the Plumbing 1-2-3 book. I think they cost $20, but if you have a photographic memory, It's free. Checkout your piping first see if it is copper, galvanized, pex,... there could be an access panel in the wall behind the shower. While at Home Depot ask the associate to get you all the parts and have him explain how to do the job.

Hope that helps,

Dan Martyn