If you discover a water line leak, the first thing you should do is shut off the water supply at the valve immediately. This will stop the pipe from leaking but also prevent you from using running water in your home.
Next, you need to locate the leak. You can usually spot it by pools of water, wet drywall, and other signs of water damage. If you are certain there’s a leak but you can’t see any obvious signs of it, go down to your basement and inspect the piping there.