First of all shut off the water to the area and see if the flow has ceased, if you still have a trickle of water you will not be able to solder the pipe. Any water flow will make the repair impossible. If you can get the water to slow to a trickle you will need to cut the pipe in half and solder a sleeve in its place at the cut. once the pipe is cut you can take a peice of bread, yes bread, and pack the incoming water side and this will stop the flow long enough for repairs to be made. The bread will break down to minute particles after the soldering is done and shouldn't hurt the system at all. If it is impossible to cut the piping at the area there is a pipe patch that you can purchase at most local hardware stores the wont require the water to be shut off however, this is a temporary repair and may start leaking again in the future. Good Luck on your repairs Hope I have helped you some

Dave