I had some of the same problems as you when I was high school and doing track and x-country, which is how I got into cycling also.
Anyway, I would say don't worry about what material the frame is made of, it makes very little difference. The most important thing is that it fits you, and that it is suitable for what you plan to do with it....do you just want to use it for cross-training? Get into bike racing at some point? Do long rides or centuries? This will partially determine what to look for in a bike, for example something with relaxed geometry (long rides, touring etc) or aggresive geometry (racing, esp criteriums), and what kind of handling the bike has.
The first step would be to get fit at a reputable shop. Better shops have fitters and/or fit systems similar to a stationary cycle so you know what size range you should be looking at.
As far as bike makers go, there's no easy answer, unfortunately (fortunately?). Mass-produced bikes (relatively speaking), like Trek, Giant, Cannondale, etc generally offer good value for money, and you can't really go wrong. In fact, since you are still in high school, one of those might be the best choice, as you don't know yet whether or not you will stick with cycling in the future. For most of the makers above, there is usually not too much difference between the mid-level and upper level bikes, except the parts spec. So you could buy a cheaper bike from them and still be getting a good frame to upgrade later if you wanted to. A good example is Cannondale, especially their alu frames....the only differences between the Optimo 1, 2, and 3 on their CAAD 9 and 8 series is the parts. Same thing with Giant. These frames may bot be terribly sexy like Colnago, Pinarello, etc, but they would probably serve you just fine.
