Before I begin, this is in no way directed towards anyone who has accidently conceived a child and has decided to keep it. My hat is off to you, ladies. This is directed instead at teenage girls that want to get pregnant on purpose.
I've felt a strong need to have a child since I was 15 years old, so I completely understand the emotion behind it. It's not always "babies are cute, I want one." but please consider a few things. If you do not pay rent, utilities, insurance, and for food, then you're not experienced at how the real world works. And when you're finally in a position to learn what it takes to make it on your own, it won't just be you... It'll be an innocent child who will be directly affected by your decisions, good and bad.
If you live with your parents, and have no immediate plans to move out and support yourself, choosing to become pregnant is selfish. You are subjecting your family to providing not only for you, but for your child as well. Babies are extremely expensive, and it's irresponsible to put that kind of burden on people who have probably put their baby years far behind them.
You should really consider what a pregnancy would mean to everyone involved. You might feel completely ready to handle having a child, but what about the father? Is he ready to be responsible for another human being so early in his life? And if he decides he doesn't want a part of your baby's life, what will you do? There are many single mothers out there who do wonderful jobs, but if you were given a choice, would you honestly choose a father who didn't want your child? Wouldn't you rather have someone that, even if they didn't want you, wanted to be a part of the child's life? Boys mature later than girls, we all know that. As much as he says he loves you, the reality of a pregnancy will probably scare him away.
Your life is going to change so much in the next few years. There is so much more out there than you know. You need to experience life, learn some hard lessons, and meet some great people. You'll be surprised how much you don't know until you look back. Don't let it be with regret. You have plenty of time to have children. The mark of a good mother is putting her child's needs ahead of her own at .A.L.L times... Including before they're even conceived. If you care about them at all, you'll wait until you're better prepared to give them everything they deserve.