Your question is so open ended, it's as if I asked you to fix my car that doesn't run, and I had no idea what kind, or anything else about it!
A picture might start to help. Water leaking from behind the wall could be oh so many things from a broken fitting, a pipe with a hole in it, a loose connection, and probably a bunch of other things.
SO, a picture, then someone can possibly direct you to disassemble it enough to maybe isolate where the leak is actually coming from, then how to fix it.
Does it leak when filling the tub, or only when you use the diverter to take a shower? Does it leak when the valves are off?