It sounds like you need to remove the floor of the shower and the tile from the walls up to at least 6″-8″ and clear everything down to the wall and floor framing (or the concrete slab, if that’s what the floor is). Then you need to install an impermeable membrane or prefab shower pan over a solidly formed subbase (usually mortar). The impermeable membrane must extend at least 2″-9″ above the perimeter of the pan behind the walls of the shower (the higher the better). If you are building your own pan, you must use a special shower drain with built-in weep holes. Once this is in place, you place your tile and grount and caulk as usual.