Leaks in your water line will cost you quite a bit of money over a short period. In fact, that is how most people realize they have a leak in the first place. The water bill comes, and the amount is several times higher than normal. Once you realize you have a leak, the first step is to determine where it is located by narrowing down the general area.
Examine the meter to determine that it is truly an outside leak. Turn off the main water valve of the house, and look at the meter. If it is not moving, the leak is inside the house. If it is moving, the leak is between the valve and the meter, or outside.
Call the utility company to mark where the pipes are located or use a metal detector. You only need to check the area of the yard where it is extremely wet unless the yard is on a steep slope, in which case the leak could have run down hill, and you'll need to find the pipes above the wet area as well.
Shut off the water, and allow the area to dry if you have time.
Turn the water back on, and find the area around the marked pipes that gets wet first, which is the general area of the leak.
Dig slowly in the area where you believe the leak is so that you don't hit the pipe and damage it further. Set the sod aside so that you can put it back in place if you don't find the leak.
Dig along the marked pipe area to find where water is coming out. Once found, shut off the water and call a plumber. Finding the leak yourself reduces the cost of repairs because you are saving search time that the plumber will charge for.