I will also put in a plug for the Boys Scouts of America. The highest rank a scout can achieve is Eagle. When applying for the military, it is my understanding that anyone who is an Eagle Scout coming into the military automatically gets bumped one rank up.
As Mackenzie K. replied, you can talk to recruiters at an early age. Don’t go alone, make sure you have an adult family member or friend go with you. You still have to be 18 to join on your own or 17 with your parent or guardian’s permission. It’s good to do your research and plan in advance. Don’t limit yourself to one branch of the military. Talk with different branches and different recruiters. Remember recruiters are trying to fill jobs that they need filled. Ask a lot of questions to find the best opportunity for you. If you are not sure, but still want to try the military, consider going part time in the Reserves or National Guard. If you enjoy the military experience, you can then volunteer to go full time active duty. However, if you don’t like it you can still be called up to go full time if they need you. Good luck!
All good advice so far. One thing I would add, when the time comes that you actually enlist or get a commission as an officer (if you go ROTC or to an Academy) make sure that anything you want is included in your contract. If it isn't in the contract, there is no guarantee at all. The only guarantee is the written contract.
This is not to say that anyone will try to trick you, but they might and this is the best way to stay safe.