Military advice needed. Which branch? How to convince my parents?

Hi, I am 17 (a Junior in High School) and pretty set on my decision to join the armed forces. I spoke with my parents about it and my dad was not to thrilled at all and my mom seemed very apprehensive. They told me I have to go to college first and are convinced that that will rid me of my desire to serve, however, I know it won't. How can I get them to support my decision with out my dad feeling that he "wasted [his] money" on an Ivy League education? My next question is which branch to go in to. I am seriously considering NAVY because I would try till the day I die to become a US Navy SEAL. However, that is not a guarantee and I am not sure, if found to be unqualified for SEALs, that I could reach my potential on a ship or submarine when I would much rather be engaging in land operations. Also, I would love to be part of a branch that will push me to all of my limits both physical, and more importantly, intellectual. I just want to be sure to choose the branch in which I can use and improve my intelligence, physical strength, and endurance. I appreciate all of your advice. God bless our troops.