I applied through a recruiter. The process took 4 months. I interviewed at US Marine Corps (Los Angeles, CA) in Jan 2011
Interview
In high school you begin meeting with a recruiter who does a quick screen to make sure that you don't meet any automatic disqualifies. They walk you through leadership scenarios where you must choose the qualities that best define what you're looking for. Assuming you don't fall out of what they're looking for you'll most likely get the job.