If you don't know what to write
STILL YOU MUST WRITE......
See, writing is not like accounting, a dumb job. Its about passion, talking and lots on it.
I think, as you are a teen, you should go for a novel or story where there are lots of dialogs are present. You see, at teens we can talk a lot (its because we are good observers at teen) and we can talk on a lot of topics. So, a story/novel with talks will be good.