[hillegom]

You need to stop water going to the valve. Then unscrew the large nut under the handle. There is a gland usually there that will stop the water from coming out.
If you are lucky, its an O ring and you need to replace, otherwise, might be a proprietary rubber seal.
First try tightening that large nut under the handle. It should be snug, don't overtighten.
After replacing the hopefully, O ring, smear just a little silicone on the rubber. Always makes them last longer.