First, turn off the mains water supply.

Normally found beneath kitchen sink. Sometimes in downstairs toilet. A brass fitting and tap with an either blue or black pipe disappearing through the floor. When fully turned off then the kitchen cold tap will only drip.  If, for various reasons, this cannot be done then:- 

Use the water supplier's stopcock. Found outside but adjacent to your property boundry.  Pavement, driveway or beneath overgrown hedge.  As a general rule, each property has its own, but not always the case, sometimes they are shared.  The middle variety twists off anticlockwise, but most can be levered up with a kitchen knife or large flat screwdriver.

Within the chamber will be a small handle or a 1/2" square knob. Turn clockwise.  Some only need a 1/4 turn while others need to be screwed like a conventional tap.  This will stop all mains water entering the property.