How do i break up with the girl i guess is my girl friend.?

OKay so i go to a cool summer camp. i will be here until mid August. i have been here for a month. We have lots of fun. it's like school without the work just teenagers. i am going to a sophmore this year. well this girl who i have been with like ually at camp but no one knows named Kristie asks me to ask her younger sister, Melanie out for this dance. She is about to be a freshman. Now Melanie is really ugly. And she is kinda fat. And she is really lame. Kristie knows this, that's why she asked me to do it becasue she knows that Mel is really sensitive and would be devestated that she didn't have a date to the dance. Well first i said know and asked Kristie to come with instead, she said know and told me she was going with this guy she was kidna seeing. she then gave me 20 dollars so i said yes. Well i asked her out and she was acting really excited and strange she screamed and hugged me. Well not even 30 minutes after i did this everyone knew. She was telling everyone about it!