First, go to the front of the house, side of the house, and back of the house, and shut off the valve that supplies the water to all the fixtures of your property. Once that is turned off, the next step is to go around the property and turn on various fixtures to confirm their water supply and drain the residual water that's inside the system. If that is not done, then there is risk of flooding and water damage. The third step would be to acquire the proper tools, which include a pair of Channellocks, a wrench, and some Teflon tape and plumber's pipe dope. Use those materials to install a new valve. Finally, make sure to turn the valve water supply back on very slowly so that there are no leaks as you're regenerating the water system.