Both HomeTips.com and The Family Handyman offer guides for fixing a leaking shower faucet, a common problem. The Home Depot hosts a guide for replacing a shower faucet.
Repairing a shower faucet requires removing the trim on the front; this task generally requires a pair of pliers to twist the trim counterclockwise. After removing the trim, those repairing or replacing the faucet can access the valve, which is the core component controlling the faucet. One of the most difficult parts of the job is applying bathtub caulk to seal the trim once it's been screwed back into place, but it's fairly simple to try again if the caulk doesn't set right the first time.