It's called an introduction. It's an opportunity to make a great first impression--perhaps the only opportunity. There's really no excuse to flounder on such a fundamental aspect of writing a story. How many times do our English teachers have to throw out the word "hook" until our mouths got the gashes to prove it? I see far too many writers go for the most boring introductions ever. Every time I see it happen, it's a lost opportunity.
Since this is a feature about fanfiction, the normal rules of writing don't quite apply. For one thing, the readers are already familiar with the characters that you're using. They may not know how you plan to spin and twist their personalities beyond all recognition, but they should have a pretty solid idea of what to expect. That's just the nature of the enterprise. We borrow another person's characters and ideas and put a new spin on it for the enjoyment of other fans. There's nothing wrong with the concept, but it does impose limits. You're restricted by the fandom's personal tolerance for deviation. How far you can push it comes down to how well you can execute it.
Writing an introduction for a fanfiction allows room for some conceit. Just saying a character's name is sufficient enough to provide all the physical description necessary to render a character in a fan's imagination. You can introduce minor variations by changing the clothes they wear, the cosmetics, and etc., but they remain largely intact. Most writers throw in a token description to satisfy the basic narrative check box, which is fine.
So my first lesson is simple:
1. Don't start off with description, especially a description that the reader already knows.
There's more than one way to describe a character besides the color of their eyes and the color of their hair. People act and react in specific ways. Try new forms of characterization that depend on thought processes, reactions to situations, and general behavioral patterns. If there's any time for using figurative speech, it's when describing a character.
2. "Try not to start off with a quote."
This is a byproduct of my journalistic days. My professors always told me, "Don't have floating dialogue." You want to establish a character before you start throwing out lines. All it takes is one sentence to establish a place and character to prep a line. Otherwise, you have a floating piece of dialogue said by nobody. But I can see the appeal, hell, I do it a lot. In those cases, just follow up quickly with some scene-setting description. The goal is to not leave your audience hanging. They should have a foothold, no matter how small, when they first read the story.
3. In media res
For those familiar with the term, it means starting a story in the middle of the action. When stories do this, they create a compelling scene that forces the audience to think, "How the hell did they get there?" After the scene ends, the story goes back to the beginning and goes from there. It's a way to pique the audience's curiosity and give them a glimpse of the future, making them more likely to stick around and see what happens next.
4. Movement Over Stillness
You don't want a static intro. Movement makes anything interesting. Don't start off with rote exposition, explaining character back story like a detached textbook. Provide those details within the framework of the story itself. It's all about scenes, and with scenes there are characters, and these characters are usually doing something. For example, let's say you have a character who's been bullied his whole life. Instead of telling the reader this, write a scene where he's getting bullied and then tell the reader all about it.
5. Why should I read this?
The introduction should answer the above question. The first thing you show the reader should be the most interesting part of your entire story. Think of it like a trailer. Sometimes, movies show all the best parts in the trailer out of necessity. You want to give everybody a glimpse of what your story is all about. Don't just tell them, show them.
To make sure that your introduction is good, read over the first sentence and ask yourself:
"Do I want to continue reading this?"
If that answer is no, then you still got work to do.
But if that answer is yes, well, now you got even more work to do.
Always put your best foot forward. It doesn't matter how great your story becomes if people never bother to read past the intro.