First - shut off the water supply under the sink.
Then check to make sure the valve itself for the piping isn't "broken" or leaking itself. If so - get a plumber. Sweating pipes (if copper) is for experts.
Then take apart the faucet, knobs themselves to see how the bushings themselves look. If you have a single "arm" - just unscrew the bolt under said arm and replace the bushings.
