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

You can ask your husband on what are her mom's likings and dislikes, e.g food, and you can buy one of them for first meetup gift. Also, dress appropriately and be respectful all the times. For me, I met my MIL for the first time 10 years ago and I bought her some mangoes as she likes mangoes. During the conversation I just went with the flow and both of us feel comfortable.