meeting his parents!😱

Hi so Im 18 and I have been with this guy who is 20, for a month and now he invited me to go to this reception with him where I will meet his parents (I already hung out with his sister and she was really nice). Can you give me some tips on what to wear, what to call them, how to not feel so nervous.... this is my first boyfriend and the thought of meeting the family has always made me really nervous.