I consider fan fiction a gateway drug to novel writing. I think many people have dabbled in fanfiction and of those a large percentage has gone forth to write novels, short stories, or scripts.
I certainly followed this path. I still have one of my first fan fictions up on this site. It is by now horribly old. It has many typos and grammar errors and the punctuation is more than just a little creative. However, I still like it and find it funny. Also, since I wrote it just for fun, those errors don’t bother me. Now let me explain why writing fan fiction is both easy and hard at the same time.
Fan fiction is easy, because many things are done already for you. When you write fan fiction, you base it on existing characters, existing stories, and an entire existing universe. Instead of digging up fundaments, you can just walk into an already built house with a decor you like and a cozy fireplace giving you warmth.
You know who you are dealing with and what kind of things are liable to happen, because you read, or watched those characters in action already. You and everybody else who reads your story know what you are talking about. You have mental pictures that are easy to latch on to.
Fan fiction is also easy, because you are just doing it for fun. You will never make a dime from fan fiction, because those characters are not your intellectual property to begin with. So you are allowed to go crazy. Flex your fingers and try out whatever you like. You just have to be very clear about it, that you A) keep it in the realm of characters only and B) don’t get too possessive of your stories. If you argue with another fan fiction writer that you used a certain idea first and therefore it is off limits for anybody else, you are clearly doing it wrong.
Another thing makes fan fiction so much fun, is your audience. Fan fiction is usually posted to message boards or specific fan fiction sites that are easily accessible online. Many people visit them and many people leave comments, suggestions and feedback. In my case, when I posted fan fiction, it usually led to instant gratification. People were extraordinarily enthusiastic about my posts. People commented with praise and specific requests. People were waiting for me to post the next installments. Reading this instant feedback was a balm for my soul.
But, of course, there is also a flip side to this endeavor, as it is with almost all things in life. Writing fan fiction can be difficult, because of the same reasons mentioned earlier.
To write good fan fiction that people will like, you have to stay within the confines of the characters and let them do things that make sense within the canon. I am actually talking more about the how than the what here. You can go ahead and put a known character in a totally weird and new situation, as long as the character reacts to it, like people expect him or her to react. For example, you can’t take an absolutely honest and open character and turn him into a vindictive slimeball for a few scenes and then turn him back to the way he was, just because it so happens to fit your story. Your audience will tell you that this won’t work. Or, to stay in the above mentioned house analogy. People don’t like it when you come into their house and proceed to hammer a weight bearing wall down.
Also, a lot of fan fiction is very erotic and some of it is simply just porn. Fan fiction can take known characters and make them do things that you can’t show on network TV, or read in a children’s or YA book. Some of the better written erotica is enticing, but people will lose interest pretty quickly if the writing is so unclear that they can’t figure out who does what to whom. Also writing and posting adult themes when you’re not an adult yourself is not a good idea. Another very important point to heed is to keep anything you write in the realm of characters. Never ship/use the actors or other real people as the basis of your fan fiction.
Lastly, the feedback and comments can get ugly. Just as much as they can lift you up, they can push you down. So you better get a thick skin before you hit send.
With these caveats in mind, why don’t you go ahead and see if you can walk a few steps in the minds of your favorite characters? Pick a book or movie or TV series you love and write a little fan fiction about it. Maybe it suits you? Maybe it is just the gateway drug you need?