Holy crap, I thought I was light. I'm 5'11 135lbs. I would go somewere like GNC or a store like that. They will probably put you on some protein shakes and things like that. On the other hand you should be eating pasta, bread and anything with carbs in it. That helps gain weight as well. I would work out the same way as normal, but eat a whole bunch more so that you don't lose as many carbs and you can gain weight from that.
We graduate at the same time, but I'm only 16 (wonderful) so I really don't have any option like that until late June of this year. I realy don't like the way you wne in. I think the best way to go in as an officer, no matter what the enlisted say. My LTC. in JROTC used a metaphor why it's so much better becoming an officer,"Enlisted men are trying to tell you that you need to know how to flip that burger. You need to know it inside out. But the thing about it is, you can go to college, look at the enlisted flipping the burger and say, 'oh, I don't need to know how to do that, I see how do it now' and your getting paied more." Some people do have that luxury, but if you have the chance to go off to college than do it, you will get a much better life.