Everything people have suggested are great ideas, however, I would also suggest you take your Ipod or Nintendo DS for the car ride, and veggg. Text your friends on the ride. In other words divert your mind.
If you feel that you do better with practice sessions, ask your parents to pepper you with off the wall questions...books, news, etc.
You need to understand yourself and what is calming to you.
Just remember they know you are 17/18 yo. They don't expect you to be able to give the answer to why there is no peace treaty between Israel and Palestine.
If you walk in, and know you deserve to be there, not in a pompous way, but self assured, they will see that too.
There is nothing any of us can say to stop your legs from shaking, except for the fact that you may not believe it, but the fact that this matters so much to you it will shine through. Just remember nobody ever died from this experience
My best wishes, thoughts and hopes.
Thank you for wanting to serve!