Before attempting to work on any appliance, make sure that all power (electricity) and utilities (water and gas) have been turned off and/or disconnected on the appliance. Coming in contact with live energy sources can lead to serious bodily injury or even death. To prevent such injuries, always remove the appliance from its power source before proceeding with any diagnostic or repair work.
It is also important to remember that some unplugged appliances still carry a shock hazard. Microwave ovens in particular utilize capacitors that store enough energy to cause serious bodily injury or even death.
The inside of appliances have sharp parts and pieces that could cut or otherwise injure you. Use caution when working inside appliances.
If you are not familiar with the diagnostic tools or are not experienced in dealing with potentially hazardous work then please contact a licensed repair professional in your area.