First, it's more likely that you have a leak in the shower riser or at the shower arm ell or connection point. But, these are often seized on, and unless you have a full set of tools and a good shutoff, attempting removal can be disastrous. Sometimes you can get lucky with penetrating oil or muriatic acid to break up the calcium/lime, but a lot of the time, you need to swap valves, unless the handle style is fairly universal and can be replaced after cutting it off of the stem.