Best thing to do is BE YOURSELF. Go to the meeting like you were going to any job interview. Be polite, be friendly, be alert, ask good questions and answer every question you get asked to the best of your ability but don't say things that are not true and don't get verbal diarrhea (meaning answer the questions but don't elaborate unless asked) you will do fine.
If this if this is a "family friend" and you have talked to him before on an informal basis so much the better. You want to be friendly, confident and go into the interview with a smile and a good firm handshake from there just relax and let the conversation go where it will. Don't try and direct the interview but follow the interviewer's line of questioning. If he asks, at the end of the interview, what questions you have - do some thinking before hand about what you want to say.
Best of luck! and BE YOURSELF!