Answer:
As a student in AP sociology I hope I can help with this.
1. Make eye contact.
2. Shake hands with her parents
3. Smile at your GF in front of them
4. Do not use ma'am or mrs. at first. Ask them how you want them to call them, and studies show that women prefer "miss" over "mrs." even after the age of 45. Try asking: "Can I call you Mrs.___?"
5. Express gratitude. "I have been wanting to meet you!" can go a long way on a first impression.
6. Wear a simple outfit. Edgy styles and logos don't always go over well. A plain white t-shirt always works great.
Hope that helps! :)