Find the control for that valve and shut it off. The first thing you would need to do is obviously replace the gasket. Easy right there. There are even instructions on the box. lolz. Just take your old one and get the same size and gauge. Then, you need to turn the valve back on and see if water is still coming out. If it is, then the pressure busted a pipe or loosened the joint. It should be controlled enough at this point to see where it's coming from. If it's a joint, try to tighten it. That should work. If it's a pipe, swap it out for another.
Plumbing is really a lot of common sense unless your bust something below your floor or in your walls.