I just found out about Inkpop, and the idea of it really appeals to me. I don't have any writings right now, though. Any help with figuring out what I'd like to do?

Off the top: If you don’t know what inkpop is, it’s a site for teens (13–18) to submit writing projects to (poetry, short stories, essays, and books), and the other teens rate and comment on it. Every month, the HarperCollins editors look at the top 5 rated projects in each category, and if they really like it, they give offer to publish it.

I just found out about this last night, and before that, I really hadn’t gotten far with writing things. I had tried, but after 3 pages or so it kind of lost the point. I think, if I get a good idea, and think of pretty much I want to do with the story, it could be decent. I’d like to try.
So here’s my question: How should I go about coming up with the idea? I know I can write better if it’s in first person. I really like the idea of a journal-type format. Help! This is pretty new to me.