Before we start, a quick disclaimer: This isn't meant to be hate-filled or confrontational in any way. Like I said, I was a young fan fiction writer once, and I did a lot of the things I'm about to go over. My goal isn't to shame, but rather to teach. By following these tips, I hope to help you become a better writer.
So you want to learn Japanese. There could be plenty of reasons why: maybe you think your future awaits you in the Land of the Rising Sun. Maybe you want to impress a cute Japanese person. Maybe you just want to watch anime without needing subtitles.