I don't know if this is something to be embarrassed by or not, but I have a confession to make: I used to write fanfiction. A lot.
For those of you who don't know, a fanfiction ("fanfic" for short) is a piece of literature that ties in with a movie, book, TV show, etc. and is written by a fan of that franchise rather than the official writer or writers. It can be anything from a short poem to a novel-length story with multiple chapters, and it usually centers around pre-existing characters from said franchise. Anyone can write one, and there are entire websites such as Fanfiction.net, Deviantart.com, and Justexpressing.com that are dedicated to sharing them.
So why is it semi-embarrassing for one to admit that they write fanfiction? Well, there are two main reasons. The first is that, as stated before, anyone can write it - including people who simply can't write. For every decent fanfic out there, there's probably a hundred terrible ones, so the practice is usually viewed as a joke among serious writers. It doesn't help that a large portion of those terrible stories are full of idiotic romance (often with fan-made characters who are obviously stand-ins for the fanfic writers themselves) and loads of disgustingly detailed smut. I've never put anything like that in my stories, and that seems to make fanfics like mine a rarity.
The second reason, which I do take some blame for, is that fanfiction tends to be a huge waste of time and effort. You spend days, weeks, months, and sometimes even years writing a story that you can't publish once it's finished because it's about characters whom you don't own the rights to. Sometimes you can change their names and make a few other adjustments to turn out something original, like E.L. James did for Fifty Shades of Grey, but normally the details are so specific that there's no way to cover up who the characters are supposed to be. I have four or five novel-length fanfics out there, and only one of them has a chance of becoming an original work that I can publish. My Kindle library could be almost triple the size it is now if I wasn't so obsessed with Lord of the Rings, Pokémon, and Pirates of the Caribbean in high school.
At the same time though, there are some benefits to writing fanfiction. If you're serious about it, it can be a great exercise tool. I can honestly say that my writing style, character development, and overall storytelling skills have improved ten times since Deer Lake, and that's thanks to practicing on fanfics and getting constant feedback from online readers. Feedback isn't something you get on a regular basis when working on a book. The only downside is that when you write about characters that have previously been established in other works of fiction, you don't get much practice at creating any from scratch. That's why it's hard to get away from writing fanfics and focus solely on original works; you get more story ideas for characters that you already know.
So the question for anyone else in this position is, how do you make that transition? How do you take that urge to write about your favorite fictional characters and harness it to help you create your own great work? The answer is fairly simple: trick your brain into thinking that you're still writing fanfiction.
For instance, if you're a huge Jack Sparrow fan, then come up with a character for your book that exhibits Jack-Sparrow-like personality traits, then picture him in their place every time you write a scene for them. It doesn't matter if your character looks nothing like him, just picture him in their place anyway. Ask yourself how Jack would react to the situation in the scene you're writing, and the ideas will most likely come to you. The goal is that eventually your character will outgrow their Jack Sparrow training wheels and take on a life of their own. You might even decide to go back and change some of their earlier scenes to make their behavior more like your re-envisioning of them and less like Captain Jack. I do this all the time with my characters, and so far, it's worked very well.
Another strategy that I've recently started to use is "reassigning" fanfiction scenes. Suppose a really interesting scene for a potential fanfic just materializes in your head one day. Instead of writing it about Jack Sparrow or whomever you picture as the subject, look at the cast in your book and decide who among them is most like the person in that scene. Make a few adjustments to the scene, and you've suddenly got a really interesting moment in your book. I took a bunch of ideas for things I'd like to see in the next Hobbit movie and turned them into what I think is a funny and heartfelt mini plot for Part 2 of my sci-fi/fantasy series. Heck, I've even got half of an outline for a Pokémon fanfic that I'm in the process of developing into a prequel to my series.
Bottom line, fanfiction can be both a curse and a blessing to writers in this day and age. The trick is learning how to rein in that wild horse so it can pull your carriage and get you rolling.