Digital multimeter

A digital multimeter is a useful tool for electricity work. There are numerous models that you can buy, one popular make is the fluke. The meter can be used to measure electricity, current, and resistance. Although there are many uses, I generally use the multimeter to check if there are live electricity in the circuit I'm working on.  Below is an image of a digital multimeter courtesy of wiki.  I use multimeter that is less sophisticated than the one shown, mainly because of the price, and the features I need for home repairs.  These meters can cost  from 30 dollars to 6000 dollars Canadian.

NOTE: ALWAYS ASSUME THE CIRCUIT IS ELECTRICALLY POWERED. NEVER TOUCH THE WIRES WITHOUT CHECKING FOR EXISTENCE OF POWER.

Electrical Safety

Electrical safety is the most important aspects of any electrical reparation. Before you start, make sure the electrical power is turned off! 

WARNING

1): DO NOT WORK ON ANY ELECTRICAL APPLIANCE IF THE POWER IS STILL ON. FAILURE TO DO SO, CAN LEAD TO DEATH OR SERIOUS INJURY.
2):IF YOU MUST WORK ON A TV, CRT OR ANY CAPACITOR TYPE ELECTRONICS, MAKE SURE YOU LEAVE THE UNIT UNPLUG FOR AT LEAST 48 HOURS. THIS IS BECAUSE CAPACITORS CAN HOLD AN ELECTRIC CHARGE FOR A LONG PERIOD OF TIME.  
3): TELEPHONE WIRES ARE GENERALLY POWERED BY THE TELEPHONE COMPANY. ALWAYS DISCONNECT THE TELEPHONE WIRE SOURCE BEFORE ADDING NEW WIRES.

There are many ways to do that, here are a few.

1. Find the electrical panel, and turn off the appropriate circuit breaker. Generally, the panel is labeled, so you will know which one is the right one. 

Here is an example of electrical panel from wiki.

2. If the electrical panel is not labeled, and chances are, it is not, then you have to do by trial and error. The way to do this is:

a. plug in a small WORKING appliance (e.g. radio) into the plug that you wanted electricity to be removed. Turn on the radio to see if it is powered.

b. Once you confirmed that the radio is working, go to the electrical panel and turn off one (1) circuit breaker.

c. Check the small appliance to see if the radio is powered. If it is still on then it is the wrong circuit breaker.

d. Repeat steps b and c until you find the circuit breaker that powers off the radio.

e. Once you find the circuit breaker, tag the circuit breaker so no one can turn it back on while you work on the appliance. `