A girl is blackmailing me

Hello, I am 14 years old. I have a friend that is a girl and she is devious. I had her on Facebook and we were talking about our day and stuff. Then she asked me,"What are you going to get me for Valentine's Day." (Take Note: she was not my girlfriend). I responded and said,"I will give you the D, and I know you want it." She asked me what does D stand for and I told her that it means dick, I then told her that I was joking but she said that I mustn't ever speak to her again. The next day we met at school she came to me with her friends and she told them what I did. She said that she won't tell anyone about the chat or put our chat on FaceBook unless I give her money. I said fine and I gave her the money the next day. But she still talks about what happened to everybody and she still blackmails me and asks for money and answers to test and stuff. She told everybody they I sent her pics on our chat but those were lies. She said that she will file a sexual harassment case against me. Please help me, she threatens me every day and I can't stand it. Everybody has a different view of me, help me, tell me would should be done, give me advice. How can I stop this. Note: she was not my girlfriend, I was just joking with her, she blackmails me all the time but still talks about what happened, she told all my friends.