Relax....
Take a deep breath and relax. They want to know about you. They are very interested in you and the desire to take this long road to a great education and career. They probably will ask why you want to attend a SA. (Probably the most difficult question). They may ask what have you done to prepare yourself. Or why should we choose you.
Remember back, way back, when you first decided, "Air Force, Yes, Air Force"?
Exploit that moment and build upon it. This is your unique story unlike any other. You must remember, the board has heard all day long, from others, they want to serve their country. But you are different, and you need to present yourself as such.
Practice. Practice in front of the mirror, dressed in you suit, tie and polished shoes. No one will laugh at you, because its just you and the man in the mirror. Then ask someone to throw question at you. Then review your answers. Ask some adults you know who can provide a mock interview.
This can all be practiced to gain confidence in yourself and being able to sell who you are to the board.
Push Hard, Press Forward