Painkillers. Take them regularly for the next few days - if you are not sure what is safe to take, either ask your parents or speak to a pharmacist (I think you have to be 16 or above to buy paracetamol and ibuprofen so you may need an adult there with you; they may be able to sell something stronger than paracetamol/ibuprofen depending on your age so definitely worth having a chat to them). Hope you feel better soon - the initial pain of getting braces is the worst, it shouldn't be as bad when you get them tightened (at least it wasn't for me).