Watch this video from This Old House to learn how to troubleshoot a leaky shower.
1. Cover shower drain with rag to prevent dropped screws from falling down the drain.
2. Remove handle from shower valve.
3. Unscrew escutcheon plate from the shower valve.
4. Inspect rear of escutcheon for water stains; check condition of gasket.
5. Remove old gasket and apply plumber's putty around back of escutcheon.
6. Screw the escutcheon plate back onto the shower valve, then remove excess putty.
7. Check all horizontal surfaces in shower stall, such as soap dishes and shelves, for cracks.
8. Inspect seams between shower stall and walls; seal all cracks and holes with silicone caulk.
9. Use a strap wrench to unscrew shower arm from wall.
10. Clean threads on shower arm, then wrap the threads with Teflon tape.
11. Thread shower arm back into wall.
12. Turn on shower and test for leaks.