Identify the Leaking Area
Check on the outside of the bathroom wall to see if you can find where in the water supply pipe the leak is coming from, which will help you find the right section of pipe to repair. Look for a telltale puddle of water at the base of the wall. If you don't see any water, feel the walls to see if they feel damp in any area. Make a mark on the wall with a marker.
Turn off the Water Supply
Because this is a water supply pipe, it will be tied directly into the main water supply for your house. The only way to cut the water going into the leaking pipe will be to shut off the water supply for the entire house. In most properties, the main water supply valve is located in the front or along the side of the house, usually encased in a concrete or plastic covering.
Empty the Toilet
Flush the toilet, which will empty water from the tank and into the bowl. Continue holding down on the handle until all the water inside the tank empties, which will help relieve any pressure inside the leaking pipe and prevent water from spilling out during the pipe repair.
Removing the Wall
Remove the part of the wall directly in front of the leak. If this is drywall, you can cut through it fairly easily with a keyhole saw or jigsaw. Start at the floor, and cut up to about 12 inches or so. Take care when cutting, and stop when you hit anything solid, like the plumbing pipe.
Repairing the Leak
Examine the section of the pipe with the leak. If the leak is at a coupling, loosen the coupling with a pipe wrench, and replace it with a new one. If the leak is in the middle of the pipe, you will have to cut out the bad section with a pipe saw or rotary cutting tool with a metal cutting blade. Measure out the cut section of pipe, and cut a new pipe of the same size. Sand down the edges of the cut pipe, apply flux and solder the new pipe in place with fittings. Allow time for the solder to dry.