If it is leaking, the problem is in the faucet, not the shower head. Is this a single handle or double handle faucet? Older faucets can sometimes be rebuilt with new washers, or you can replace the old cylinders with newer ceramic disk cylinders that won't need to be repaired again. To fix this, you'll probably have to turn off the water to the house, as most tubs/showers don't have their own shutoff valves. Carefully, with a knife or small screwdriver, pry off the cap at the center of the handle, remove the screw from the center of the handle and then remove the handle. This may be stuck on and may not survive the removal process. Sometimes, Lime Away, or some similar product, can be applied to dissolve the calcium buildup. Keep rocking it back and forth till it breaks loose. Sometimes a puller can be used to remove it, but this usually messes up the handle. The trim under the handle needs to be taken off, unscrew it counter-clockwise. Now, you'll probably need a special stem socket to unscrew the cylinder. Before you unscrew this, turn on the faucet to make sure that you actually did turn the water off first. Unscrew the cylinder and then take it to your local home improvement center and match it up to what they have there. They will have repair kits and/or replacement cylinders. Repair or replace the cylinder and then replace it from where it came. Replace the trim and handle (either the old ones or with new ones). Screw the handle back in. Replace the trim cap. Turn the water to the house back on, and then test the results.