Is it leaking out of the handles? Or is it leaking out of the shower head/tub spout?
Usually the only time you have to replace a stem is if it’s bent, snapped off, or stripped(will endlessly spin clockwise)
If it’s leaking out of the handles, 9 times out of 10 you can just tighten the packing nut(or if your hard core you can remove the old packing and put new packing in and tighten the nut down).
If it’s leaking out of the sput or shower head then you need to replace the rubber(or neoprene) washers, washer retaining screws and the valve stem seats(which requires a seat pulling wrench)
I hope this helps.