I've got an idea for a story but just don't know how to start it...please help?

Maybe you could start out with how the girls meet the boy, introducing why they want to kill him. The unwilling girl should be sorta taken with him right away, lost in his eyes or something like that. This is a good basic plot, but it should go more in-depth. Perhaps, if you are planning on having the girls keep and torture the boy for an extended period of time, the nicer girl could have secret meetings with him. At the end, the mean girls could find out and decide to punish the nicer girl by killing him, with her as witness. The mean girls drop a hint that they knew about the girl and boy's love, and, in agony of betrayal and loss of her love and herself, the nice girl kills herself, thus joining her true love in heaven.