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

Just be yourself, you can ask your husband that if there are any specifics you need to follow. But I agree be casual, you are not going for an interview. Just be polite and respectful as you are with your parents. Just don't do this :D https://www.youtube.com/watch?v=p2U49xHFOFc