You becoming a vegetarian doesn't stop the animal slaughter... It'll be there, but you'll be covering your ears to it by not eating meat, eh? You're not making a statement, you're thinking you are. And if your parents don't support you, you're already halfway to giving up on it.
But yeah, basically what Odlov said - protein. It's mostly what meat has that you need, and you'll need to acquire it from other foods.
(As much as I find being a vegetarian stupid.)