If you try to shut the water off to your fixtures by using an angle stop and that doesn't work you will have to shut of the water main to make your repair. There are also many other reasons you might need to shut the water main like repairing frozen pipes, repairing leaks , replacing fixtures or faucets.
Many people are not aware that they even have a water main valve right in their own house and just call a plumber of never wind up doing the repair until things get really bad because they simply do not know that shutting the water main is really easy.
The most common places a water main will be located in home is in your boiler room, basement , or a cut out panel in a section for your home.
Once you find the water main you will simply turn the valve clockwise to shut all the water off in your home until you have finished the repair. Some homes will have a ball valve with a lever that you can pull instead of turn to shut the water off.