I personally couldn't wait to drive, I had wanted my license since I was a little kid and I got my permit when I was 15, Vermont allowed us to get a permit at 15, I think they still do. I wasn't scared at all to drive, though I had quite a bit of non car practice beforehand. I drove golf carts ever since I was a young kid, and I think that helped quite a bit. When I first got my license I was fearless, though now having driven for almost 10 years safely with probably about 300,000 miles under my belt (I drive a lot of miles) I see too many reckless teens out there who are dangerous. I was one of them and I was lucky to never crash, which I attribute to my superior driving abilities - yes, I'm one of those people who thinks they're the best driver's on the road. However, in fairness, I've taken a number of high level driving courses, both track racing and winter driving so I am pretty good.
I guess what I'm trying to say is that it's really a good thing that you're a bit apprehensive, it makes you more alert and a safer driver. The best thing that you can do is to feel for yourself how the car will handle at its performance limits. Anyone can drive in a straight line slowly. Find an open road with no traffic or better yet a huge empty parking lot. Floor the car from a stop (if you're driving a real car with a standard tranny, don't forget to shift!) Slam on the brakes from near highway speed and I mean STAND on them, you need to know what the car is going to do, try cornering quickly at a decent speed, you need to know how to handle this and how the vehicle is going to react, though do NOT attempt this in an SUV. Oh, and buckle up. You will be able to drive more safely and confidently if you know how your car is going to react when pushed to it's limits. Also, keep in mind your road conditions and the type of car you are driving, a Porsche is going to handle a lot different than say, a Chevy Caprice.