Just sit them down and tell them that you are going to serve our country in the Air Force. You'll probably need to explain why you are choosing this career path since us parents want to know EVERYTHING!!! Hopefully they'll be supportive from the get go, but if not just give them a little time to get used to the idea. I honestly don't see why any parent would be anything other than proud of their child for choosing such a career.
When my husband told his patents he joined the marines was by packing his stuff An hr before he had to leave. His parents were not supportive then. I suggest you just sit down with them and tell them this is something that is in your heart. Good luck! Military life is hard bur can be very rewarding!
Tell them you have decided what you want to do with your future and have set yourself up for a life long career. It's a great career path, be confident in it and they will see your happy with your choice and will hopefully be supportive. Good luck!