Is there a guide to writing Fanfics somewhere? How to make them be the best they can be and everything? Or even better, a guide to writing HP Fanfics?

Also, I'm writing my own fic and I want a reasonable reason for Harry to get powers, that nobody or not many people have used before.

 

www.lumosdissendium.org is alright I suppose...

http://forums.darklordpotter.net/viewtopic.php?p=46884#46884

But I'll tell you the best possible teacher, good ol' experience. Your first fic (or the first few chapters of your first fic if you're good) won't be what you wanted it to be, but you should just keep plugging away and you should improve.