It's Always Hard to Begin Writing a Story.

I always have had trouble with this. I never have any trouble coming up with a story that I should write, but I can never put it onto paper.
Wanna hear the idea?

Sunny is the fifth of six children in her family. Meaning, she's usually the one forgotten. The youngest always gets special attention, but nobody remembers the quiet and introverted fifth. Her father is a smart, successful businessman who expects all of his children to get great grades, but Sunny always falls behind; thus her father pays no attention to her. Her mother is in the hospital for leukemia and is the only other person she can connect with. But when she begins to be bullied by her ex-friends at school on top of her mother suddenly getting worse, her world is turned upside down.
Then she met Seth. She'd never met someone who's life was so much harder than hers, and yet was so happy. Slowly, Sunny begins to fall in love with him and begins to see hardships his way. She starts so see how she's not the only one hurting; there are people all over the world with problems worse than hers. She learns how she can do just little things and make a difference, just like Seth has. And she finds the reason Seth has such unfailing hope in his life.

What do you think? Any ideas how to get started? I've been mulling over the first chapter all summer and haven't gotten anywhere.