Fun, great posts above.
Assuming the OP isn't another scam new account designed to yank chains I'll try to also help.
OP, seek to understand WHY your family is opposed to you serving. Is it serving or serving in the army that is the issue? Why?
Talk to your family about your interest to serve.
Explain to your family that you as a top priority want to maintain lifelong/ great relationships with them.
Explain that you can walk away from ROTC without commitment as a college programmer, but that you want to try/ see if it is for you.
Posts are correct that you are 18 and it is your choice, but being cut off from your family may not be your goal. I know too many people who no longer talk to their relatives - usually over inheritances and such, but it does happen. It's sad.
Good luck.
Just for giggles, my threaten to be disowned experience was a little different. Flush with cash from mowing lawns (I think I had like 40 bucks - living large!) When I was like 12 I bought a poster of actress Heather Thomas from a store called Spencers and hung it in my bedroom wall. Momma Herman Snerd was most not pleased, and threatened to disown me if it wasn't gone by the time my Dad got home. She was like go get a poster of the Dukes (bo and luke) or a sports team. So I went back to the mall


, and bought a shirtless Eric Estrada poster and put it up instead. My Dad came home, asked what was up when he saw Ponch from CHiPs staring at him, suppressed a laugh, I briefly explained and he said "put the first one back up" and that was the end of that.