A Ground Fault Circuit Interupter is almost as important in your home as a fire extinquisher. These receptacles will help to save your life in the case of an electrical short by shutting down the power to the recepatacle when it senses a short.
Make sure the power is shut off at the breaker box before you begin. Remove the old receptacle and seperate the white, black and ground wires. If you find theres 2 blacks, 2 whites, and 2 ground wires in the box, you will need to find which is line and which is load. Line is the black and white coming from your breaker box and Load is the black and white wire coming from the receptacle box to another outlet.
If you don't have a meter, Plug a lamp into receptacle and remove one black and white wire combination and put electrical tape or wire nuts on these wires.
Turn the power back on and if the lamp comes on the wires attached to the receptacle are the line wires. If the lamp doesn't light then the wires that you taped up are the line wires.
Turn the power back off at the breaker box and remove all wires from the old receptacle.
Most GFCI receptacles will have a tape across the load wire screws.Attach the two load wires to the receptacle (black to bronze, white to silver).
If you had 4 wires in the receptacle box now is the time to attach the load wires, Remove the tape covering the screws and attach the same way as you did the line wires.
After installing the GFCI into the receptacle box. Turn the power back on and test the GFCI by pressing the test button on the front of the receptacle
If you have an older home that have no GFCI's it is a good idea to install these in the bathrooms and kitchen area