Glad you asked...
The symptoms of a leaking shower are usually caused by worn or damaged seals in the valves that control the flow of water to the spout or shower head. When making this repair, it is a good idea to take the parts from one or both valves with you when going to purchase replacement parts, as there are many types of valves on the market, and they do not interchange.
I have found a site for you that describes the repair procedures for the most common types of valves. Hope this helps. You can find more info at:
http://www.fixafaucet.com/...