My first ever go at writing..?

try deviantart.com or quizilla.com. You can get instant feedback from other writers there, and discover other stories. Try making a plot map, so you know what will happen (the key to jk rowlings sucess!). Make sure the audience knows what you're talking about, and try shorter stories before long ones. You probably won't become a bestselling novelist before you're 18, so don't rush. Use this time to work on your story. And don't give up!