Recent stats reflect that 75% of American youth are NOT qualified for the Armed Services. That's due to a lack of minimum education, mental health problems criminal record, and physical disqualifications. So finish school and be physically fit, that should be obvious and easy enough. Study your Service of choice. Know the history, rank structure, essential subjects, organizational structure, etc. That's what you can do to prepare. Here are three things for you to know: (1) Military service is government service. Government service is extremely inefficient. That's where the phrase "hurry up & wait" comes from. You will be doing stupid, inefficient things and the best way to handle that is to LAUGH. If you STRESS about it, you will suffer terribly; (2) The primary purpose of the military is to destroy the enemy. You might work in logistics or disbursing, but the organization you work for is in the business of killing people in defense of the country. Embrace that concept; (3) Your experience in the armed forces is what YOU make it. If you have a terrible time, it's because you made it that way. If you have a great time, it's because you made it that way.