Hey Brian, I was in your shoes just a few years ago, applying for an NROTC scholarship, now I am a Midshipman at Boston College. (Although when I had my interview for NROTC it was with a Chief, I wonder why yours is with a Captain.)
The main thing I would say is try to get your GPA up. I'm assuming you're going into your senior year so you won't have much time to do this, but if you can get it above a 3.0 that'll help a lot. My GPA in high school was about 3.3, technically not that much better than yours, but the selection boards are really looking for something in the 3+ range. That's what I'd say is your weakest point right now but it doesn't automatically mean you won't get a scholarship. I would also re-take your SATs and try to get them above 1300--mathematically it's only a 20 point difference, but psychologically it's a bigger difference than that. Your sports and extra-curriculars look great.
As far as "talking to new people", this is a skill you will have to acquire. Not just in your selection interview, but in NROTC and the Navy itself, and for most jobs in the civilian world, you will constantly be interacting with new people. Remember that when you are commissioned you will go to a flight squadron or a ship and it will be your job to lead enlisted sailors, many of whom will come from very different backgrounds than yourself. You will need to be skilled at "talking to new people" (superiors, peers, and subordinates) in NROTC, in flight school, and in the fleet. In the long term, there are a lot of things you can do about this--most people naturally become more socially interactive as they go thru college, and if you still need help you can even take a speech course in college. For the short term, the best advice I can give you is to BE CONFIDENT. Be humble, but be confident. Walk in, look whoever is interviewing you (you will probably be interviewed by 1 or 2 people in addition to the Capt.) in the eye, give them a firm handshake, call them "sir" or "ma'am", answer whatever questions they ask you clearly. If they ask you if you have any questions of your own, don't be afraid to do so (it shows eagerness). I was interviewed by a Chief and another enlisted sailor (seperately), they asked me questions like why did I want to be in the Navy, what do I want to do in the Navy, what do I have to offer the Navy. I was asked one question that I was completely unprepared for and it took me a little while (probably only a few seconds but it seemed like hours) to think of an answer--sorry I can't tell you what it was, but it would've been easy to answer if I'd been able to prepare and I can't take away all your fun.
If you have time the day of your interview, lift some weights and go for a run--not only is it good for you, but you'll look better (lifting weights makes your muscles temporarily swell up and this makes you look more impressive) and feel better. Then watch a few scenes of "Top Gun" to get yourself fired up.
Bottom line, just be relaxed (don't smoke, it won't help you when it comes time for PT!), be confident, be humble and respectful but BE CONFIDENT. The Navy and Marine Corps don't want officers who are going to second-guess everything, who can't make up their minds and who look unsure in front of their troops; they want officers who make a good impression, lead by example, and are decisive. In many situations, ANY decision is the right decision--the toughest part is making it.
If you don't get a scholarship, it's not the end of the world--go to a college with NROTC, join the College Program, and you can apply for a scholarship (most College Programmers get them by the end of their sophomore year, many by the end of freshman year). But the best way is to have a scholarship, and in order to get a scholarship you need to go into your interview with confidence.
If you have any other questions feel free to shoot me an e-mail at fowleral@bc.edu, otherwise good luck.
GO EAGLES!! (Boston College and Philadelphia)