From the corrosion, I'm guessing this has been leaking for a while. If it's just a bad threaded joint, then you just need to unscrew the fittings, starting at the closest union, clean the threads, reapply some pipe dope to the male threads, and screw them back together in reverse order. It's a pretty easy repair requiring a couple of large wrenches, a wire brush, and pipe dope. If you have an inside shutoff to just this line, it's very DIY.
If the valve is leaking from the stem, and that's a 1/4 turn shutoff, then you should simply replace it. These tend to be very nice valves, but there isn't much to repair when they fail.
If a soldered joint is leaking, then the pipe should be cut and soldered again with new sections of pipe.
Unless this has been leaking ever since the installation, I'd guess it failed from freezing. Make sure these lines are fully drained in the winter, or wrap them with heat tape and insulation, to prevent freezing and damage in the future. And if you don't have an inside shutoff to this line, you really should get one added, along with and a drain line.