How to clean a microwave?
You can put a cup/bowl of water(you can also add some vinegar if you don't mind the smell) in there, heat it up to boiling, let the steam out. Then you should let it sit in the microwave without opening the door. 
Wait for a few minutes, open the microwave, and wipe off the inside. All that steam should have soften any stains or food pieces left in there. 
You may spray some natural cleaners or lemon/vinegar mixtures to wipe off the inside more.