Here are a few things nobody told you yet, but these should help.
Remember to smile. Not a goofy grin, but a nice smile when you meet him -- even though you have met him before, and know him a bit. Shake his hand. Strong grip, not like you're trying to mash a hammer handle though. Look the man in the eye when you speak to him. He will know you are being sincere and are interested. If looking them in the eye is hard for you to do (shy people find this very very hard) then look at the bridge of his nose. The gentleman won't know the difference, and it helps overcome shyness. (Try this out tonight when you talk to your mom or dad. It's better than trying it out the first time in the interview) Have a couple of good intelligent questions to ask. You'll only need one or two. "What are some more job responsibilities you would like me to do?"
Answer his questions honestly. If you don't know an answer to a question, don't try to BS your way out. Simply say, "I don't know that, but I'd be willing to learn, or find out, or whatever."
LISTEN! If he is telling you something, listen and look interested, even if you feel your eyes glazing over. If you look interested in getting a job, a lot of times you'll get hired even if you're young -- just because people want to hire someone who is interested in working for them and doing a good job.
If you have an opportunity -- usually toward the end of the interview, tell him "I'd really like this job. Or, I'd really like to work for you. I know I will do a good job for you." This can often clinch a job for you. Good luck to you, I'm rootin' for ya!
Cheers,
JimDon
PS remember to smile!