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

Just be yourself. Be polite and don't overdo things. If you put on an act, she'll see through it eventually. Perhaps you could get her a nice gift? Doesn't have to be very expensive, but something she will like. Ask your husband for suggestions. I met my MIL when I was pregnant and she as very kind to me. I wasn't very nervous and things turned out well.