Don't forget to turn off the water first.
There should be a rubber washer on that type you replace at the bottom of the part you remove by unscrewing the big hex nut like thing you will see once you take off the knob and covering under it. Hardware stores and the building stores sell the rubber washer seals. A small screw normally holds it in place. If you get it apart and its different post a photo and someone will advise or take that whole thing to the hardware and they will show you how it works.