Firstly, howdy neighbor! <--in Memphis!
Secondly, this is kind of a loaded question. Lol I would definitely recommend going through some of the articles and forum stickies on this site, because there's a LOT of really good information on what to eat, what to drink, best ways to get motivated/started, etc.
Just to hit some of the highlights that worked for me:
1.) Drop the carbonated drinks, period. No excuses, no exceptions. There's not a single one out there worth drinking if you're out for that kind of body. If you're like my little bro' (14) and into the energy drinks, chuck those, too! Furthering that point, start drinking more water. If you don't like the taste of water, there are some ways around it. One is to put a bit of lemon and/or lime juice in with it to give it a citrus taste. SpecialK also makes a powder that's high in protein and fiber that you can mix in with water to give it some flavor.
2.) Start brown bagging your lunch to school if you don't already, and start doing it wisely. Start packing fruits, sandwiches made with turkey or chicken and whole grain breads (there's all kinds out there). You may also want to consider keeping a bag of stuff to keep in your locker or backpack for stuff like small cans of almonds and other nuts, raisins, etc. to snack on throughout the day between your classes. If you can, try to spread your caloric intake across the whole day, rather than taking it all in at lunch time.
3.) Cardio. Start walking, outdoors if you can manage it, on a treadmill if not. One thing I always did in high school was keep a brisk walking pace even when I was in school, for all the times that I couldn't be out on a bike or running. If you don't like running or otherwise can't run, try rollerblading or biking. If you have access to a gym, elliptical machines are also excellent. Most people recommend doing cardio 3x a week for at least 25-30 minutes per session.
4.) Sleep more. At 18, you shouldn't have any problems getting lots of sleep anyway -
- but if you don't, I would advise you correct that. People underestimate how important sleep is to trimming down and bulking up. The body does a LOT of repair in your sleep; depriving it of that time only hurts you.
5.) I'm no PT, so I would suggest you do some research before you just take my advice on weights. If you've never really worked out with weights before, I'd start easy. Stick to low-weight, high-repetition routines (ie, shoulder press 15-20 lbs 15 times, rather than trying to shoulder press 35-40 lbs 5 times). There's a lot of good information on different workout routines on this website. I'm actually going through the muscle building one myself, and I'm really liking it so far.
Whatever you end up doing, just go easy at first, figure out your own limitations, and start working through them.
Hope this helps. As I said, I'm not certified or anything. I just know what has worked for me. Do your homework (for class AND this!), and pay attention to what your body tells you. If you pick up a weight, and it's all you can do to keep a grip on it walking back to the bench, you might not want to try to hoist it over your head. If you're stretching or lifting or whatever, and you feel something pop, or tighten, or otherwise feel out of place, stop for a minute, stretch a bit, and maybe do something else. The last thing you want to do is hurt yourself.