The light in microwave doesn't go off when door is closed ?

One of the two or three safety switches that tells the microwave its door is shut has failed - or the operating tab on the door has broken off. Unless you know EXACTLY what you are doing (and, based on your question, you won't) then don't risk the lives o fyour family by attempting a repair yourself. get a techie to do it or simply get a new microwave.

XX
August 2015

There will be a crafty mechanism that senses that the door is closed - a small switch very probably. It will be defective.

Your appliance needs to be seen by a specialist - either that or you consider buying a replacement.

Good Luck...
August 2015