What's wrong with short stories? I've written a few. They're good practice - get a storyline going in a short story, and get your descriptive techniques and whatever to the point and your larfin'.
Just sit down and start writing about whatever the hell you want. Put the ideas down as they come along. Once you've got a few ideas of the same type together, you can flesh them out into a full story.
What're the short ones you've written like? Care to post your best?
I thought you were 16... but I'm not going to say anything.
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Am I allowed to ask if you'll e-mail me a couple of your stories, then? If you don't want to, then I can understand. I know from personal experience that people don't like others reading what they've done. Heck, I've got loads stored away that no-one is allowed to set eyes on... yet.