Surprisingly enough, I wanted to be in the RAF. Up to about 15 I was really keen, although my interest was waining the more I discovered music, girls and other such hobbies. I had interviews and went on careers weekends with them etc.
I don't know if its the same now but I was going for a sponsership through university to eventually become a flyer. I guess this went on for a couple of years until one day I got a letter saying I couldn't join up because my Dad was Hungarian which at the time was behind the Iron Curtain. I was gutted for a bit but soon realised it was for the best.
I'm pretty glad it never happened although I do wonder what my life would be like now if I'd signed up.
My advice would be get your A Levels go to College and see how you feel after that.