You need to replace the bib washers--the rubber washers located on the bottom of the hot and cold stems. Once you do that, you won't have to turn your handles so much to shut off the water.
1. Shut off the water supply to the tub.
2. Remove the handles. If they don't come off easily, you need to buy a handle puller from the hardware store.
3. Remove the sleeves and escutcheons surrounding the stems.
4. Using plumbing sockets, remove the stems.
5. Replace the bib washers.
6. Put everything back together.