As a new writer, the first thought of some people is that their story is going to be filled with riveting characters, plot twists, amazing and imaginative new things and that everyone will want to read it.
The truth is that the first story is going to be at least a little painful. I can speak from personal experience, because I've written many of awful things.
You don't want to bog yourself down with an overly large world. I think this is why some people are genetically predisposed to writing fan fiction based on their favorite books, movies or games. The logic goes like this:
It's actually quite a lot of work to invent a universe by yourself. It's much nicer if you can take an existing universe and just move in. Like a land squatter. You can even use your own characters.
Imagine that I really liked Harry Potter (which I do). If I liked it enough, and wasn't feeling particularly creative, I could start writing fan fiction inside the Harry Potter universe. I could make my own students attend Hogwarts. I could make up a few additional teachers, and they could have grand wizardly adventures together.
I'm not trying to speak out against fan fiction here, as I've read some (actually quite excellent) stuff by a handful of not-published internet writers.
The problem with most fan faction is almost like my adverb argument: This kind of thing sets you up for bad habits and failure.
In my new Harry Potter story, assuming I'm only the most casual of writers, my characters are probably going to take the form of the existing characters from those books. Lets say I loved Ron and Ginny. They're who my characters would be. All that I have to do is change the names and I'm golden.
My main character is, naturally, going to be either very much like Harry or very much like my own personality. This is called a self-insert or Mary Sue. I make Harry-me (or “Hairme”) talk to my version of Ron and Ginny the same way I would personally talk. Ron and Ginny, in turn, talk to Hairme the same way I would expect my friends to talk to me.
This is a problem. At this point, I'm no longer writing a story; I'm writing what I would like to do in Hogwarts. This is just me hanging out with my friends, but with magic. It's a vacation; a fantasy. It's not what I set out to do. What happened to my compelling characters? What happened to the plot twists and exciting villains?
The problem might be that the basic structure of the story was too close to the original work. Harry's (probably) already done all of the things you set out to do, and worse still, he did it better. Instead of innovating a story, I've only created a bland retelling of it.
To make things more interesting, instead of students at Hogwarts, I could make my characters be muggles outside of Hogwarts who have finally realized something is up. I could bend the lore a little and explain what happened to the Unplottable spell on the school. Or maybe I could tell the story from the perspective of a teacher. Or from someone who actually didn't get into Hogwarts and is forced to find another means of education. Or set the story in one of the other magic schools mentioned in the original books. Durmstrang, anyone?
You don't need to make your characters have the same experience as Harry. In fact, if it were really my story, I'd steer clear of making the characters anything like J.K. Rowling's. The problem I foresee is that Rowling had so many minor characters with so many personalities that it would be hard to invent a new person without them seeming a bit like someone from Potter canon.
These are the sorts of reasons that I always avoid writing fan fiction if given the choice. Not only are you limited to the kinds of stories you can tell (Lord of the Rings fan fiction would certainly fall prey to this), but you're always in danger of retreading old ground. If you make your own universe you're completely free to do whatever you like.