I accidentally put aluminum foil in my microwave, how can I clean the scorch marks?
Check your microwave's owner's manual for a cleaning procedure and follow it. 

If there isn't one, I'd try the procedure recommended by Samsung which I've summarized below: 


Sprinkle baking soda over the interior of the oven and rub the entire thing with a damp cloth. Wipe clean with damp cloths.
Make a 1 : 8 vinegar to water solution by volume (at least 130 mL of this stuff) and microwave until its boiling rapidly. Leave in oven for 15 minutes then wipe down with damp cloths.
Use acetone (in many nail polish removers) on a cloth to remove scorch marks. Wipe clean with damp cloths. 
Put some instant coffee powder in some water and microwave it until your microwave doesn't have an acetone smell.


You may have to repeat step 4 a few times; basically, just make something that smells pleasant a few times so the scent of acetone goes away.