What are the steps for fixing rotting sheetrock behind a shwer wall?

Water somehow must of gotten up behind the shower wall and caused mold or mildew behind the caulking. When I went to remove old caulking put re-caulk, I found black crap coming out from under the wall. And then wet sheetrock started coming out. I have a feeling its all rotted and bad back there. Unfortunatly this is our only shower and don't know anyone, personally, to fix it. Someone pleas help!!!