Well, you cannot enlist until you are 18, or 17 with parental consent. However, there are many youth organisations that are military in structure and will help prepare you for a military career. Check and see if your school has a JROTC program, you could also look into CAP and Young Marines.
Many of our younger posters here are members of these groups and I am sure they will be along shortly to answer any questions you have about their respective groups.