I have not met my mother in law, going to meet her soon. How should I behave for good first impression?

Do your homework, ask your husband or partner about his mother and while you should be yourself and not be fake, try to avoid topics and actions that might set you up in a negative light. Whatever intel you can get from your man will be useful haha. I remember meeting my future MIL for the first time, after a 12 hour drive from one end of Sweden to the other. I was exhausted but I remember applying lip gloss in the car and having breakfast with the family and mustering all my energies to maintain a great conversation despite my sleepy state. It worked well (I think) heehee.