Is it a Moen brand? They are quite common. They use a cartridge inside. You have to replace the cartridge. Get the new cartridge, shut off the water, and follow directions that come in the pack. Also get some plumber's silicone grease to lube it as you insert it.
If you end up with more problems, post them to the PLumbing Forum at www.handymanwire.com where it is easier to communicate.