“Fanfiction (also abbreviated to fanfic) is fiction about characters or settings from an original work of fiction, created by fans of that work rather than by its creator. It is a popular form of fan labor, particularly since the advent of the internet.”
The best authors I know, admit they started writing in fanfiction. As much as some see it as some sort of taboo. Perhaps worried about not looking professional if they so much as hint it.
Writing is writing! Words written down will do nothing but improve your skills in an art form that you love so dearly. I started in fanfiction… Ronin Warriors to be exact and from their I branched out a little but it is still a good part of the writing I do today. The large following, great places to post your work, and quick gratification does nothing but encourage me to write more.
I tend to write AU (alternate universe) anyway, so any stories I create can live on once new characters had been mulled over and used as replacements.
Be PROUD no matter what you are writing. Practice, practice, practice!