Advice on replacing bathroom fixtures
You are replacing what is known as the trim. The part behind the wall is the shower body. Most manufacturers have a bunch of differant trim styles for each shower body they make. The way the trim attaches, spline counts, and other factors differ from brand to brand and often what fits one shower body made by brand X won't fit another shower body made by brand X.
If you happen to have the information on the shower body (brand, model number, etc) it should be easy to find new trim. If you don't you can take what you have to a plumbing supply house and they may be able to help you out.
If you bought a real cheap body at Home Depot chances are slim of finding replacement parts for it.