Jenn,
They're your parents. They love you, unconditionally. Simple answer: you tell them. Find a quiet family time and bring it right out, "Mom, Dad, I'm thinking about going to West Point for college and becoming an Army Officer. What do you think?"
What will be their reaction? Who knows? I'm assuming shock will be pretty high up there ("What! Our little girl wants to join the Army!!??"). Probably a little fear as well ("In a time of war!!??"). But most likely, the first question out of their mouths will be "Why?"
Be honest with them. Give them your honest feelings why you want to join. Expect their reactions to those feelings (again, most likely shock and awe). But most importantly, DO YOUR RESEARCH before you talk to them. Have some answers to some basic questions. Better yet, ask them to look up the answers to the questions you all will have together with you. Starting here on these forums is a terrific place to search for that knowledge, for you and for them.
There are a LOT of parents on here as well as cadets, all of whom have little to no military experience. They have been in the same boat as you (guess i should say Tank. If you want a boat to share, the USNA link is right below this one). There are also a LOT of really great individuals on here willing to listen (OK, read) your questions and give you honest advise and feedback.
Just my two cents, but nobody is going to have your interests at heart more than your parents. They will always be your best guides in this. Get them involved. Talk to them.
"Mom, Dad, I want to go to West Point....."