Before you join the military, and while you're in if they'll let you (in the US they often pay for members to take graduate degrees), and afterward as necessary, take as much science and/or engineering as you can. Try to get at least one post-graduate degree in a field related to something you would do in space, whether a practical degree such as medicine or engineering, or a science like biology or chemistry. Two is better, but regardless pick stuff you like because you need to get top marks.