First of all,locate the wall studs and then take a framing square or a level and mark a rectangle slightly larger than the damaged area(center your vertical lines on stud centers to allow an area to fasten the patch,then score the lines w/a utility knife and cut it out w/the knife or a keyhole saw."Clean-up" the edges of your cuts and along the horizontal edges of the sound drywall(if needed)fasten furring strips half behind drywall and half exposed,for an area to screw to.Then buy the drywall to match the thickness of the existing wall,cut and screw to your studs and nailers w/ 1 1/4" coarse thread drywall screws(sink the heads slightly below the surface).Then apply mesh tape to joints and finish w/joint compound(usually 2 coats),sand,prime and paint!See that's no so bad..oh and one more thing..since this is in a bathroom I'd recommend you buying "MR board"it's moisture resistant and great for humid areas.Have fun w/your project!