Warnings
Never operate a microwave if the door is damaged or doesn't close securely.
Don't heat water or other liquids beyond the time recommended by the manufacturer or any recipe. Superheating can occur when plain water is heated in a clean cup for an excessive amount of time. The water will look innocuous, but when moved it can literally erupt out of the cup. Don't heat the water twice - that adds to the superheating risk. Adding sugar or coffee granules to the water will reduce the risk of superheating.
Don't use metal containers unless the recipe specifically directs you to: as stated above, microwaves bounce off metal, which can cause arcing and a fire inside the oven. Some recipes may call for shielding parts of the food, especially meats, with small amounts of foil. This is perfectly acceptable as long as the directions are carefully followed.
Don't operate the oven while it is empty. This can also cause arcing and start a fire.
Make sure any glass, plastic containers, and plastic wrap you use are labeled microwave-safe.
Plastic type 5 is the best type of plastic for microwaving, and most containers labeled microwave-safe are type 5.[1] Plastic types 1 and 2 may also be safe if labeled as microwave-safe, but some caution against letting the container come in contact with food when heated.[1]