Gossip Girl:
From what I've seen of you on TSR you seem like a bright, articulate girl with lots going for her. The world is your oyster. You could do A-levels, go to uni, or leave education and get a good job. You can do anything you want to do, and if you don't like it you can change your mind. If you have a baby now, you can't change your mind later. If you have a baby now, you won't be able to afford to pay for holidays, a car, that pair of fabulous shoes, or anything you really want, because all your money will be spent on nappies and bottles. You won't be able to give your kid everything you want to, because you won't have as much money as the parents of other children, who waited until they were older and settled. The chances of you staying with the father of your baby are minimal, which means you'd be struggling to bring up a child on your own, and unlikely to get a new boyfriend who was OK with the fact that you had a baby. It wouldn't be fair to bring a baby into the world under those circumstances, no matter how much you loved it or how competent you were at caring for it.
You are a person in your own right, and you have your whole life ahead of you. If you have a baby you will become someone else's mother, and you'll have to sacrifice yourself and give up any selfish pleasures for a long time. You will be jealous of your friends who will all be out doing "young person" things. When you start your career, you'll be dragged back by the fact that your mind will always be half on your child, and you have to make childcare arrangements and take time off work when they are sick.
Please go to your doctor and talk to them about getting the injection or the implant, so you don't do something silly and throw your life away. You have a lot of potential and shouldn't waste it. You'll be fertile for about another twenty years. Plenty of time for babies later!