First, try to tighten the handle with a screwdriver(there is usually a phiilips head screw on top of the handle). If that doesnt work, get a couple of crescent wrenches and tighten the top nut while holding the bottom part of the fitting with the other wrench. Usually that will solve you leak. You should probably turn off your water before doing this so you dont get water all over the kitchen if you brak something and to take off the pressure going to the valve. The ball valve sugestion is a good one too, if you are up to a bit of plumbing.
Be careful doing this though: You could have plastic piping going to the water valve and if you crank down adn slip off the valve you could break the line down in the wall, which would really suck. I cant tell if you do by looking in the picture, so that is why I suggested to turn off the water before doing this.
Peace
Lounatik