Adrian, good on you for making a plan for your future. I started investing when I was just coming out of high school and it definitely was one of the key things that set me up for success. If you are interested in the Air Force, I strongly recommend you keep that as an option. There are not many better deals than an ROTC contract. You will get a free four year college degree, a guaranteed job with benefits, and some pretty cool life experiences. Not to mention, the USAF is consistently rated one of the employers with the highest employee satisfaction. My military career gave me the capital I needed to start investing in real estate, as well as setting me way ahead of my peers in terms of income and responsibility.
If you are currently fifteen, it sounds like you are way ahead of the game. You should definitely focus on finding your first job, and doing the best you can when you get it. Every employer you meet will be a reference for your next position, so do the best you can to maintain a positive relationship with your bosses. Save your money, and once you have a minimal amount saved up, start investing. The compound rate of return over time will be what makes you rich. These forums and the BP podcasts have a great deal of knowledge. Good luck.