What can I do if someone has falsely accused me of sex with a minor?

Go speak to a criminal defense attorney. Do not post any information about your situation on any social media sites. If the cops call you, or want you to come down to the station so they can ask you a few questions, neither talk to them on the phone nor go to the station without an attorney present. If you say anything to them, even something that to you may seem innocent or by way of explaining that you did nothing wrong, you will most probably incriminate yourself. You have a right to remain silent and a right to have an attorney present before any questioning – assert those rights! Which means again, that you should talk to an attorney as soon as you can.