If they serve you meat, refuse to eat it. If they put steak on your plate, take it back and grab another plate. At 14 I was telling my parents that i don't eat meat, and at the start they said no, but once they realised that I wasn't going to change my mind, they put up with it, and eventually came to accept it.
You may need to argue to make your point, but at 15, your parents shouldn't controll every little action you do, you should have some freedom. And if you need to, tell them its your religion

(i think buddhists don't eat meat, but don't ask me i'd have no clue lol)
If your parents serve you spaghetti with meat in it, don't eat it. make another pot of spaghetti, cook some chips, make vegan pancakes (allways fun

) or vegan biscuits (also very fun, especially when you dye them sickly green colours)
Make a stand, if you must, go without food for the night. Once they realise that you've made YOUR decision that YOU are not going to eat it if it was once moving, they will realise that by preventing you from becoming vegetarian/vegan, they are starving you.
Also, when I first became veg, I just picked the meat out. That way my parents didn't see it as much of a hassle. If we had fried rice, i'd pick out the bacon, because that way my parents didn't have to cook in a separate pot. Now that they've come to realise that I'm not going to change my ways, they cook food for me in a separate pot (i will never eat anything that has touched meat, except chips from the fish and chip shop), or I cook dinner (for everyone) which means that 4 or 5 nighs a week we're not eating meat, because mum's too lazy to cook a seccond pot/tray of food, or i'm cooking dinner. The other nights i'm at friends places cooking for them (or they're cooking for me, which is even better because they're teaching themselves to cook veg) or i'm at home being lazy. Even the sunday roast happens about once every two months, instead of every sunday.
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