I've seen fanfiction writers get the hard end of the stick. By that, I mean that others won't hesitate to make fun of a fanfiction writer just because they write fanfiction. Even if the writer doesn't write anything inappropriate, they still get made fun of.
If you're not sure what fanfiction is, the term basically has the meaning in its name. It is short stories, or longer, that a fan writes based on their favorite fandom. Anything can have fanfiction. NCIS, Harry Potter, anime, just name it and there's a 99% chance someone has written fanfiction of it.
All I gotta say is, what's the harm? In fact, do you realize how beneficial it is?
Aspiring writers need all the practice they can get. In today's modern age, coming up with an original idea is so tricky. Sometimes writers feel like everything has already been taken. We don't want the lack of original ideas to stop us from writing. So, some of us like to borrow characters and plot that we enjoy while not taking credit for it.
"Wait, how in the world do you do that?" you may ask? Fanfiction!
Fanfiction is a wonderful practice tool for writers who want to practice their skills without the stress of coming up with something on their own. They can post their fanfiction for free and on their own time, and get feedback from fellow fans.
In fact, there have been many authors who started out writing fanfiction. Ever heard of the popular romance novel series Fifty Shades of Grey? That was originally a Twilight fanfiction. And now the author is probably rolling in cash from the box office and book sales her story has gotten.
If you are an aspiring writer who enjoys even just one type of media and you also need practice, write a story based on that thing you're into. It can be a movie, a video game, it can literally be anything! Hell, do what Tina from Bob's Burgers did and write "friend-fiction." Whatever it takes to get that practice in!
If you need help writing fanfiction, there are tons of resources I have seen on Tumblr and Pinterest with tips and tricks on getting started. The best websites to post your fanfiction are Tumblr, Archive Of Our Own, Fanfiction.Net, Wattpad, or your own blog! Share it and tag it with the fandoms the fanfiction is about.
An author once told me to "write to learn." So let your imagination go wild, because you never know how much it can help you in your future writing career.