How do other people know your age? I don't see it anywhere... anyhow: The biggest tip I can give is: Don't post a story idea immediately onto AFF, but let the 'meat' get marinated well before you eat it. If you get what I mean. How many times I used to post new stories and never finished them. For some I never ended up writing one single chapter. My rule now is to write at least 3 or 4 chapters to try out if I actually enjoy writing my storyline and always write down a certain 'problem' for the start of your fic, which the main character must go through. And always write down your plot, so you know to what ending you're going to work your way to.
