Feedback and dialogue are crucial for a healthy community, and even more important for the author. Having your readers happy, engaging, supportive, excited, and loyal is the dream of any author. Perhaps even more than the fanfic itself.
Here are some tips on how to at least increase the quality of your community.
Be polite, and understanding, and explain your circumstances to them. Dialogue is key.
If someone is rude, ask them if they had a bad day. You would be surprised at the answer you might receive.
Make your fans feel special too. (cosmopolitan)
Even if you only have 20 at first, interact with them. Respond to DMs. "It doesn't take much time to engage," says Gardner. "They're so happy when Anna does something as simple as follow them back. She'll respond to their DMs, they'll take screenshots of it, and send it around to their friends. That type of acknowledgment is very important to fans." Before you know it, you'll have 200 followers, then 2,000, and so on.
As said previously before, put the tags, and warn your audience of the type of story you are making. Don't betray their time. It's the only thing that's truly finite in this universe.
Don't engage haters. Engage criticism. Praise those who help you and remove those who don't. Be mindful of free will though. The internet is free, let it be free.
Learn to take criticism. (DeviantArt)
Criticism will help you improve, but sadly, not enough people can take it. If it's a flame, yes you can be mad. But if it's true criticism, take it as a compliment that the person took the time out of their day to try and help you. Everyone respects the beginner who knows that they can improve, more than the elitist who thinks their perfect and refuses to try and improve.
You need to start getting to know your fandom intimately. (wordmothers)
Learn the little jokes between members and the memorable moments the community shares. Find out the episodes that are most treasured and recalled, the rules of "shipping" / creating relationships between characters, and even the history of the world in which the story takes place. You need to do these so that you can immerse yourself in the fandom you're writing for.
Furthermore, these will help you write the scene or reference that the readers will remember 20 or 30 years down the line.
If you're not from the fandom yourself, lurk around the dedicated forum threads and get close to the regulars. Then, ask them questions whose answers will help you create a quality story. Don't just frequent the big fandom communities; visit the small ones as well. In the latter, you're more likely to encounter fans who aren't afraid to criticize or ask obscure questions.
Interact with your potential readership. (wordmothers)
Generally, social media is a place where you can connect to your reader base unhindered, but keep in mind that it is also a competitive turf. To grow your social media brand, you should be consistent, easy to target in searches, and good about giving back to your community through free advice and valuable content. Many fanfiction sites also have a way to add an author's note or a comments section which allows you to develop a close relationship directly with your readers. If you build that relationship, this can be a great fanbase that you can transfer to future original works you create.
If you're concerned about staying true to your reader base, you might choose to create your own online community. Many authors and brands have their own official Facebook pages and/or Telegram groups, and others open up their fanfic for fan edits.
Prove you're a true fan by incorporating Easter eggs. (cosmopolitan)
You need to show you're part of the "tribe" about which you're writing. Keep up with the news surrounding your universe and slip references to those moments into your plotlines. When One Direction released a video of the band working out, for example, Todd set the next chapter of her series in a gym, that way she could describe Harry (renamed Hardin in print) looking the way he did in his IRL footage. "Those types of Easter eggs are like a secret language."
Now, let's talk about the author's note. (tvtropes)
Never insert these inside a story; it breaks the feeling of immersion and distracts the reader. Any meta notes should be placed at the beginning or end of a chapter. Make sure they're distinct from the story so that the reader's eye can just skip past them easily. (this is also true for meta comments the author makes mid-chapter about his own work).
If knowing a certain fact is vital to understanding the fanfic, usually you can introduce that fact within the story, not outside it.
It can be tempting but don't spoil the story in the Author's Notes (although it's okay to say things like "If you don't like horrible gay rape, don't read this story").
Don't make them long and tedious. Don't feel obliged to reply to your reviewers. If you want to do that, there's a PM system. Readers don't care about your holiday in Majorca, even if it did keep you from putting a chapter up. And if you took a time out, don't waste your time writing a long and boring apology, just write the damn story.
Some people break the fourth wall and joke with canon characters in the AN, telling them horrible things will happen or complaining about their roles. Unless your story is meant to be funny to some degree, don't do this as it shatters immersion and drama, lightens the mood, and gives the reader the constant nagging suspicion that they are missing the joke.