The first thing to remember when dealing with a plumbing emergency is DO NOT PANIC. You should familiarize yourself with where the Main Stopcock is situated, there are normally 2- one underneath the Kitchen sink and a Further one on the outside of the property. Here is what the main stopcock could look like.
If you discover a burst pipe or water falling from the ceiling, turn off the main stopcock underneath the sink, if that for some reason has seized turn off the main stopcock outside of the property!
If it is a Tank, cistern or toilet that is leaking you may be able to isolate the water independently to the main supply.
If you cannot isolate the water supply you can Flatten the pipe so it is closed on the supply side of the pipe as long as the pipe is Lead or Copper.
Place Buckets beneath the Leaking water and turn on all the taps in the house, this will drain the system faster. If you find it is a burst pipe then you have several choices-