Seal a leak a. If the leak is small, go ahead and attempt a repair. If it is really large, start the attempt, and call a plumber as well. Then, find where the leak is. If it is a small break in a pipe, you can use sealant to fill the hole.
Plumbers use two-part epoxy.
This can be bought at a repair store. Just mix it up according to the directions on the package, place the epoxy on the leak, and let it dry. When it is dry, you can turn the water back on in that area, or in the whole house. For a larger leak, take a rubber hose or rubber sleeve and place it on the leak. Then, take a hose, or pipe clamp, and clamp it to the area.