Representation is incredibly important when writing a story. Representation allows different people to see themselves in a character. It's a chance for someone to see someone like them as a hero.

Representation can mean a lot of different things. And you have to ask yourself, what am I trying to include? Is it the LGBT? Racial representation? Religion? Body types? Masculine women? Feminine men? Gender in general? Age? Disabilities?

In order to represent something, you have to know it well. If you want to make a character with autism, but you don't have autism yourself, don't write your story around their autism.  Even if you've researched it well and know a lot about it, you can't truly understand what it would be like to have autism and you may risk including myths into your representation. You can have characters with autism, but if you don't have it yourself, avoid expressing it from their point of view. Don't make it about their autism. Simply make them a character who has autism, and let them be human.
(Also you can just hint at it but not explicitly say it so it feels less about their autism and more like the character is just a character.)

Writing LGBT is the same. Don't write a book about being lesbian if you're not lesbian yourself.  But don't be afraid to have lesbian characters!  Avoid stereotypes, they can be harmful. Here are some ideas of representation the LGBT wants to see, and some things to avoid:

-The "gay best friend" is an ugly trope. It's in just about every 2000s romcom ever and it just feels like they only exist to be a token character. (Aka, not a good thing).

-Have queer characters in healthy, good, long lasting relationships.

-Not all kids of the LGBT have unhealthy relationships with their families. I see a lot of people make their queer characters have homo/transphobic parents and then abuse them with that. It's ok to have characters with bad family dynamics, but remember that's not how it always works out in real life. Coming out to your parents doesn't mean you're now kicked out. That happens a lot less than you think.

-Don't make their angst (if they have angst) revolve around them being LGBT. Unless you know what it's like and you're writing from experience, this is something you should avoid.

-Binders exist.  If you have a trans boy or non binary umbrella character (AFAB), it's good to allow them to get a binder. Making them use bandages or duct tape is cliche, and may give unhealthy ideas to a reader. Now yes, trans people AFAB will use bandages and tape in real life, but.... Don't use it for every character. It's really not healthy.

-Have aromantic and/or asexual characters stay aromantic and/or asexual.  Having them fall in love in the end is almost like your saying aromantic and asexual people don't actually exist.

-Have queer people be treated with respect.

-Don't sexualize your queer characters. In other words, if you're a straight cisgender girl writing about two gay men, don't make it sexual. And certainly don't write about them having sex because it's usually incorrect and makes queer people very uncomfortable.

-Transgender characters don't just magically transition. It takes time. It's a long and expensive process. 
(On this note, I'd like to say that I am in fact a trans man.  It really does hurt to see people make their trans characters just magically transition. I've seen someone have their fifteen year old trans boy character already have top surgery and testosterone and be fully transitioned. That hurt. So you have to be aware that 1. Transitioning is expensive. 2. Children usually can't transition and if they do they HAVE to have parental consent. 3. Kids can't get surgeries until their 18 I believe. 4. Testosterone/estrogen takes time to become affective and it varies from person to person.)

-Queer parents!

-Normal queer people that don't have a tragic backstory!

-Queer people who don't die!

-Gay men aren't always feminine. Lesbians aren't always masculine.

What about characters with disabilities?

-Don't make their personality be them having a disability. In other words, if a character has ADHD, that's not their personality. Being hyper, distracted, etc is a personality.

-Don't make their angst revolve around their disability unless you have that disability and can write from experience.

-Let them be human. Don't treat them like they can't do anything. Autistic people aren't stupid, don't just give them the mind of a child and treat them like one. Seriously, let the autistic character be important and strong in your story.

-Research the disability if you don't have it yourself. This avoids writing stereotypes and myths about it. I have a blind character and I've done an excessive amount of research on being blind. If I hadn't done that, I wouldn't have known that they make devices to put in cups that beep when the glass is full so that blind people can pour their own drinks or fill pots of water to boil. (And obviously I learned a thousand other things that's just an example).

-Allow it to affect them but don't let it be the only trait about them.
Ex: A character who has ADHD struggles in school and making friends because of how they behave and perceive things, but they're also a really cool and funny person who loves the 80s aesthetic and styles their hair in an odd way because they think it looks awesome.

-Don't make their disability be the only thing they talk about. It's just annoying.
Ex: A character who's missing a leg and only ever talks about how they're missing a leg. "I can't go swimming guys, I only have one leg." "I've always wanted to be a track star, but I'm missing a leg." "I can't walk like you guys do." Etc. For one thing, amputees can do a lot more than you think. And yes, they can swim and run track. They're not a human ornament. For another, it's almost burdening to read.

-If it's a physical disability, it doesn't have to have some tragic backstory. People are born with deformities.  A character can be missing an arm simply because they were born that way, not because they lost it in some tragic event or accident. People can be born deaf. People can be born with paralyzed legs. It doesn't have to be tragic.

-Again, RESEARCH IT!

I'm going to talk about transgender representation now because I have experience with the topic, and I didn't cover everything in the LGBT section.

When writing trans characters:

-Treat them like human beings.  They're people. They're not just trans.

-Mention that they're trans casually. It doesn't have to be some big reveal or some dramatic event.  It can simply just be a single sentence. "Hey who's the boy in this picture?" "That was me as a kid." "But you're a woman?" "I know."

-Trans people can exist in any setting. They're not just some modern day "item". If your story is in the 1800's and you want to include a trans character, then do it. Because they existed back then just as much as they do now.

-Research things about transitions. Even if they're trans yourself. Binding, tucking, hormone therapy, etc.

-Don't be cringy about it. "I was once Robert... now call me... Roberta."

-Don't fetishize them. I've been fetishized. It's not funny, it's not cute, it's not flattering, and it's transphobic.

-Don't include unsafe practices UNLESS it's plot important or for character development.
Ex: Trans character cutting themself because of dysphoria or other problems in their life... but they grow to love themselves more and learn how to care about their body and needs. That's good for people to see, it brings a lot of hope to us.

-Don't say something like "He's trapped in the wrong body" because I swear, everyone uses that. It's too much.

-Don't write that one scene where a friend discovers their friend is trans because they walked in in them changing/showering. Because... why??? That's really stressful for the trans character and may make readers uncomfortable.
(Granted, I have a scene in one of my stories where one dude needs to go pee and my dumbass trans boy didn't lock the door so gets caught trying to flatten his chest. But before that, the dude had thought the trans boy was a girl, and so it basically gets him to eventually admit he thinks he's trans.)

Racial representation:

-If they're from a certain culture and we're raised in that culture, they probably still practice it.
Ex: Japanese boy still speaks Japanese even though he moved to America and can speak English.
Ex 2: Cuban boy still celebrates Dia De Los Muertos.

-Following up the first pointer, just because they're a certain race, doesn't mean they're from that culture.
Ex: Mexican/Hispanic boy has lived in America his whole life, doesn't know any Spanish, doesn't celebrate Mexican holidays, doesn't have parents/grandparents who lived in Mexico, doesn't "act Mexican", etc.

-Characters can have mixed races.

-Black girls being feminine and cute vs strong and gonna whoop your ass because black woman power. Aka, don't just continue what the media has fed to you. Feminine gentle black girls are real.

-Describe them as being pretty or attractive (if fitting for the character) because often, characters of certain races are described as "big boned" "unattractive" "odd/weird looking" and it's bothersome.

-Don't slap a stereotype on them based on their race. Oh, so you think all Japanese people must be short and petit?  Meet Iisan Takanashi, my Japanese character who's 6'3 and may actually just be a tree.

-Depending on their family/where they live/if they are culturally accurate to their country and what not, give them an appropriate name.
Ex: Bradley (my character) is Hispanic but has a very non Hispanic name. However, the rest of his family does: Pilar, Marquesa, Santiago, etc.
Ex 2: Iisan (character previously mentioned) has a Japanese name, as does the rest of his family: Haruka, Tsuki, Makino, Tajima, etc.

-However, their race doesn't mean they have a name like that.
Ex: Jason (my character) is Hispanic, but no one in his family has Hispanic names because they're fully Americanized and have nothing to do with their ancestors anymore: DJ, Avery, Joan, Cassie, Cam, Dori, etc.

-Their personality isn't their race. Black character is an actual character with a life, personality, interests, flaws, goals, hobbies, etc. he's not just the black character and you didn't create him so you can say you included representation.

-Black/African does not mean they're ghetto.

-Being Hispanic doesn't make them automatically a tequila lover, or ghetto.

-Being Asian doesn't make them super smart and a perfectionist.

-Being Indigenous doesn't mean they know what tribe they come from, or are tree huggers.

-There's so many different heritages, and ancestral roots are important to a lot of people. Saying a black person is just African when they have a home country within Africa, is sort of dumb. (Africa is a continent, not a country.) It would be like saying a white person is only European. 1. European natives aren't all white and 2. There's different countries white people are native to. So if a white character can be British, Norse, Scottish, Irish, etc, then an Asian character can be Korean, Chinese, Vietnamese, Japanese, Thai, etc.

-Cultural appreciation, not appropriation.

That's all I have for this. There's a whole lot more I could go on with but I'm ending it here. If you have any questions or more to add, leave it in the comments!

But most important representation of all: HAIRY WOMEN