How can I tell my parents I want to join the Marine Corps?

I am 17 years old and a junior in high school right now. I deffinetly want to get my diploma before I join and will be 18 before I graduate but I really would like my parents to support me. I have talked to my mom about it before but she thinks I am joking and anytime I say I am serious she asks me if I want to die in war. I would die if I needed to for my country but do not wish to die. My step-dad was an MP in the army and doesn't think it's a bad idea but it's not his signature I need to enter the DEP. And also does anybody know if your need both signatures of your parents or just one to enter the DEP?