Hmm...You got a good basic plot already. I would suggest trying some exercises on Nanowrimo to begin. As for how to begin it, I would suggest beginning with the classic "lets lay it out" such as
"Her name was Veronica (I like that name, its my grandma's, and that's how she looked and acted when she was younger), she wasn't one of those curvacious cheerleaders the jocks dated, but she still could out-flip any of them, that was what was important. She had Aryan looks, something that both annoyed and excited her, she had a wide variety of oppurtunities and life, and took to them with pleasure, her attitude attracting friends she loved like sisters, though their feelings were not similar, they still remained close to her.
Despite her looks and bubbly attitude, she refused to be told off by friends or cheerleaders alike, winning her the title of 'Fashionita' among them all. It was this alone that led the player of the school to her, like a hound to a rabbit."
Uhm...whoops, went wild with my example. If your taking my advice, just start it up and keep writing until you reach an end, then go back and edit it. (Basically, outlining the plot then making it better)
Again, sorry for writing an entire intro into my answer.