There's a nut under the faucet handle where the stem goes through. Rotate this nut counter clockwise with a big wrench. This nut has to loosen up and be removed. Once removed, rotate the handle until the whole assembly comes out. look at the washer that's held in by a screw. remove the screw and take the washer to Home Depot or Lowes and have them match it. Take the whole assembly with you.
Good luck... Jasper