The problem lies with either the shower floor and walls or the piping. To isolate the problem, flood the shower with a garden hose or a garden sprinkler. If it does not leak, then the problem is with the faucets or showerhead. You will have to open up the wall behind the shower and check the supply piping. The problem is usually between the faucets and the showerhead, so open the wall closer to the showerhead. If it does leak, isolate its location. Allow the area to dry, then carefully run water down the drain, without touching the floor. If it leaks, you will know that it is the drain pipe. If you have determined that it is not the drain pipe, use the same test covering the floor of the shower with water. If it leaks, the problem is the joint between the tiles and drain.