The air vents at the rear of the oven must be kept clean and free of any obstruction.
A blocked air vent will cause the oven to overheat, and cut out until it has cooled down again. If this happens repeatedly it will eventually damage the magnetron.
There are usually some vents under the oven as well, these are equally important to the 'well-being' of your microwave oven.

Clean the door and the cooking area (cavity) of the oven with hot soapy water, and a rough cloth. Never use abrasives, they will damage the lining of the oven, especially an enameled finish!

Cleaning a combination oven is more difficult, and should be done immediately after use. You will need to put in a lot of effort to get it really clean. You can try using a large heat proof bowl of water mixed with washing up liquid or a domestic cleaner such as flash (never use an abrasive cleaner, you will damage the finish on the lining), set the timer for 10 to 15 minutes, and allow the mixture to boil. When it has finished it's time, allow to stand for about 5 minutes, then clean the oven with a rough cloth using the remainder of the water. It's a good idea to protect your hands with gloves, as some household cleaners are not too kind to your skin, and also the water may still be hot enough to burn your hands, please be careful.