How can i go about telling parents want to join the navy?

i have really made up my mind out joining the navy. but my mom is really against it. i even thought of not going just to please her. but it is something that i want to do. i want to go into the dep program next year because i will be a senior. but i no she wont sign and let me do it. so how can i convince her that she should sign and let me go into the dep program?