How old are you?
When I annonced at 16 1/2 that i wanted to join the Army, my dad went nuts, his words were "You will go to Northern Ireland and get blown up you fool, anyway, you cant join without my permission as you are too young". When I told him that I would wait until I was 18 and join without needing his permission he saw that I would join up no matter what, and it would be better with his blessing than without. for trhe first time in my life (and the last) I had a grown up conversation with him, he was ex national service and hated every minute of it. I told him about all the fantastic careers that the Army had to offer, and the opportunity to travel.
Try telling your family all the positive aspects of Service life, good pay, conditions of service, better food than most 4 star restaurants (honest, especially in the RAF). Emphasize that you will learn a trade, self discipline and that you will be putting something into the community that will keeop on growing when you leave the RAF.
Good luck with your future career