You didn’t say where the leak is occurring, but I’m guessing it’s in the stem under the valve handle. That’s the most common place for leaking to be a problem, unless the valve itself is failing at its job of closing off the flow in the pipe. (In that case, either the valve needs replacing or the system needs cleaning – and maybe straining / filtering of the flowing media – to prevent debris from preventing a 100% closure of the valve.)
So if the leakage is occurring at the valve stem, then what you’ll want to try first is tightening the gland nut by ¼ turn at a time (don’t go hog-wild here) until the leakage stops. The gland nut is the brass nut around the stem in your photo. (Remember: ‘righty-tighty’ – turn the nut clockwise as you look down on it through the handle.)
If turning the nut doesn’t solve the problem, then you may need to replace the packing. This is a job that should NOT be done while the piping system is pressurized. You should turn off the flow upstream from this valve and drain the system before you loosen the gland nut to remove the old and replace new packing material.
Any capable hardware store clerk can advise you on the packing you need if you show them your photo – and advise them if your system is for anything more than water.