Old taps may become difficult to turn on or off. Or you may find not much water comes out.This can be because the valve is worn out or scaled up.
You'll nee to remove the valve similarly to changing the washer (see fixing a dripping tap.)
Once you get the valve out inspect it. If it's got a lot of lime scale on it it might be worth trying to remove this with lime scale remover and finally grease it with silicon grease (available from plumbers merchants) if this doesn't help simply replace the valve (approx. £5).