Summarize the following:
Fanfiction is always based on an existing work of art. You are essentially making a story that expands or otherwise alters existing fiction. The mediums you might choose from are endless. Fanfics have been written about books, movies, television shows, video games, and just about anything else with a narrative base and a fandom to admire it. You should pick a fictional universe you already feel closely to. Common choices for fanfic are Star Wars, Harry Potter, and a number of anime and manga franchises. The choice of universe you choose to base your own work on will have the largest impact on your story and the way it turns out. Certain universes also favour certain approaches in fanfic. It's important to note however that your choices as a fanfic writer are limitless. You can do anything you want to the source material, even if that means transforming it into something else entirely. Most fanfiction tends to be based on a science-fiction or fantasy-themed universe, like Harry Potter or Star Trek. These are great to base fanfics off of because they offer huge universes with an endless amount of storytelling potential. Look online and read up everything you can about the given universe. Even if you're planning on breaking free of the established canon (what the author/director/etc has already established) with your fanfic, it helps a lot to know the rules before you break them. The best ideas you'll get for your work will be inspired by the source material itself. With that said, it still helps to see what other fans have done with the same ideas. Using a website like Fanfiction.net, Archive of Our Own, and Wattpad, or you can take a look through some fanfiction that matches your on source material. Read some stories people have written. Most importantly, get a grasp of the ways in which people use and adapt the source. In looking for fanfiction to read, you may get the looming impression that a lot of fanfiction lacks for quality. Being part of the fanfic community means acknowledging that not everyone is at the same level of skill. Most fanfiction is amateurish, and a lot of it frankly isn't worth reading. It takes patience to find the great stuff.

Summary:
Choose source material to work from. Read up on the fictional universe. Read some fanfiction.