I just stopped eating meat all at once when I was 15. I don't eat beef, pork, poultry, or seafood (I've always hated seafood anyway). I do eat eggs on a biscuit about four times a year, if even that, and I drink milk in cereal only. I eat cheese, but I know I shouldn't cuz it's so fattening. Don't worry, I get all of my vitamins.
My mom's a health freak, so she makes me take these multi vitamins that have every single vitamin I could need in the exact amount I need for every day. They're made specifically for young adult females, too, so you know I'm getting exactly what I need. I also take, in addition to the multi-vitamins, Echinacia, extra Vitamin C, and extra Calcium and Magnesium.