Your stop cock is usually under the kitchen sink, in a downstairs cupboard, or sometimes outside in a pavement box. Turn it clockwise until it stops. This cuts off the water supply and stops the burst pipe from flooding the place while you wait for us.
If the burst pipe is part of your heating system (feeding a radiator or near the boiler), turn the heating off at the programmer or boiler. Don't try to drain the system yourself unless you know what you're doing, but stopping more hot water circulating helps.
If it's safe to do so, mop up standing water with towels or a wet vac. Move furniture, rugs, anything valuable out of the way. If water's coming through a ceiling, stick a bucket under it and poke a small hole in the lowest point of the bulge to let it drain controlled rather than bursting.
If you're going to claim on home insurance, take photos of the damage before we start repairs. Most insurers will cover the water damage to your home (carpets, ceilings, walls) but not always the cost of finding and fixing the pipe itself. Worth checking your policy.
If you've got a serious flood, open windows and doors to get air circulating. This helps dry things out quicker and reduces the chance of mould growing after we've fixed the pipe. Don't use electrical appliances near standing water.