If you're not okay with birth control, you really need to abstain from sex. If it was truly God's will for you to get pregnant, he'd find a way to get you pregnant. Remember how Mary conceived Jesus?
You need to protect yourself. An IUD will protect you from pregnancy for years! This is time you can spend on your education, getting into a good college, establishing yourself in the world. You can put money aside, move out on your own, build your credit, buy a house, settle down... and when the time's right for you to have a baby, you'll be in the best position possible to give your baby everything he or she deserves. Can you honestly say you can do that right now?
If you got pregnant right now, it wouldn't be because it was God's will. It would be because you were too stubborn to listen about birth control and you put YOURSELF in that situation. I think deep down you know that. You need to stop and think about what you truly want, and then think about what your baby DESERVES. A good mother doesn't put her wants above her child's needs. A good mother doesn't choose a poor future for her child on purpose.
One little piece of plastic can not only protect you from pregnancy and give you a great shot at a bright future, but it'll ensure that you won't bring a child into this world before you're prepared for one. It's the most loving thing you can do for your future baby - give him/her a strong, confident mother who's actually able to take care of him/her without relying on others to help her.
Listen to your mother.