A set of common shower valve wrenches available at any hardware store should do it. Remember, there is a smaller, outer nut which is just the packing gland, Do not remove this. You need a larger wrench which slides down over the larges part of the body of the stem assembly.
Make sure to turn off main water before starting.
Make sure to open the stem partway before trying to unscrew.
Once you get it out, take it to the parts store for an exact replacement, and make sure to get a new seat as well as the stem.