I know a lot of YA writers, myself included, who write in first person. The problem with first person is that it’s cemented very deeply in your character’s mind—the book is very theirs, their words, their voice. So everything in the text has to feel authentic as something the character would say.
Which is kind of a problem if you’re writing, say, sci-fi, and your character stops to suddenly explain the recent apocalypse and why everybody has white hair and is on a spaceship. That’s something your reader needs to know. The issue is that your character already knows it and is about as likely to start spouting off about it as an American would be to pause mid-thought and go, “And then the Founding Fathers signed the Declaration of Independence, and…”
Some great advice I heard recently was to use your character as your lens. In other words, since your story is filtered through your character, the best way to share information about your world is through details that your character picks up.
So maybe instead of “That building was established twenty years ago as the acid rain management headquarters”, your character could think “Sarah’s working at Acid Rain Protexx HQ today—I wonder if she still wants me to bring her lunch.”
Except with better writing.
When we walk around and go to the grocery store and brush our teeth, we notice things that we don’t even notice ourselves noticing. They’re so familiar to us that they don’t stand out. To your character (unless they’ve been dropped into an entirely new world), their world is familiar in the same way. But their natural observations about their surroundings can be used to key in the reader on the way the world works, because those observations will stand out to readers who are unfamiliar with that world.
Try carrying a notebook around with you for a day and writing down things you observe – things that, if your life were a narrative being read by someone who isn’t used to the way this world works, might clue someone in on the universe. I did that recently, and it helped a lot!
The important thing is to practice a natural, in-character way of telling someone about your world without actually having your character give a lecture.
Little details can go a long way and often can flesh out a world much more than a sit-down “this is the way things are” paragraph.