You’ve made your character. You know what race/class combo you’re using, and you know your stats, but other than that, who are you? Who is your character that you’ll be playing as in your upcoming Dungeons & Dragons adventure?
This question can go in a million different directions, and all of them are great. For instance, between the various books in the Fifth Edition of the game, there are so many questions you can answer about your character, and these are all excellent jumping off points. I think they all lead to one of two things, which we’ll explore in a minute.
This is easier if you create your character narratively, which I’ve written about here before. If you take a look at each stage of your character’s life, than it becomes much easier to figure out the backstory questions below. However, if you just want a barebones backstory, this guide will help.
All of these are like empty spaces in the Mad Libs that is your character’s backstory.
In the traditional Player’s Handbook, there are spaces like Personality Traits, Ideals, Bonds, and Flaws, as well as Appearance and Background. These are all excellent things to think about for your character. If you want to go the extra mile, you can even roll on the Trinkets page, and have a very random item that you now need to incorporate.

If you go check out Xanathar’s Guide to Everything, it really begins to flesh out your characters backstory in specific ways, like family members, and why you chose the background and class that you did, as well as additional items and attributes specific to those classes. There are even Life Events and Tragedies that might have happened in your past.
Then, in the Eberron: Rising from the Last War book, there are two excellent additions that ask why you are adventuring in the first place, these are the Debts and Regrets. What things have you done in the past that haunt you, and that you are trying to either hide away, or remedy?

As you write these, it doesn’t matter if it’s more narrative and flavorful, or just a series of bullet points (in chronological order). I find it best, either way, to stick to around 1-2 pages, because anything too short and you might not have answered enough questions, but you also don’t want it too long, because your character should be relatively new at being extraordinary. If you have already accomplished so much, you wouldn’t be level one, and you might have all of your coolest moments behind you.
All of these are just ways to help you answer two questions that need to be included in your character’s backstory.
What do you care about, and what answers are you looking for?
These two questions are because your backstory is for your Dungeon Master just as much as it is for you. While you want to get to know who your character is, your DM wants to know how to make the game/adventure/story you’re about to play mean something to you. We want this story to matter to you, so we need to know what we can include that will make you care.
What Do You Care About?
This is the first major thing your DM is looking for in a backstory. You can find these in your Ideals, your Bonds, your Flaws, and your Regrets. Why did you become an adventurer, why did you choose your class? Who are the people you would want to defend? What revenge are you trying to achieve? What curse are you trying to break?

On Reddit, u/JimBaby created the “Knife Theory,” which covers this section really well.
When writing a character’s backstory, it’s important to include a certain number of “knives”. Knives are essentially anything that the DM can use to raise the stakes of a situation for your character. Anything that can make a conflict personal, like a threatened loved one or the appearance of a sudden enemy. They’re called “knives” because the players lovingly forge them and present them to the DM so that the DM can use them to stab the player over and over again.
Reddit

While this might seem not great to new players (“Why do I want to make it easier for the DM to hurt me?”), again this is just about trying to find things that will move your character to action, beyond just money.
If the DM wants to show that the villain is very evil, will you react more strongly if he assassinates the King, or burns down a small village? Every character is different, and what they are willing to fight for is different. If there is an army that’s about to attack a neighboring Kingdom, are you more likely to help if it’s Orcs or Drow? These are the things that you care about, so the DM can make sure that your character wants to get involved in the first place.

What Answers Are You Looking For?
The other, equally important aspect of your character’s backstory, is about the information your character wants to know. This is often overlooked by backstory advice on the web, because people know that DMs love to hurt their players (which we do, it’s true). However, sometimes we are stuck on what to do next. You killed the dragon and saved the princess, got your reward, and…?

What’s next? What is the next adventure that your DM should start lining up? Well, if there is a question that your character wants to know the answer to, then that could inspire your DM for the next story.
Whatever happened to your grandfather’s sword that he lost in the Mines of Mror? Where is your long-lost brother? Why is your family heirloom glowing? Who in your family’s past made a deal with an evil entity, that has now made you a sorcerer? These are all very important questions that you don’t need to answer. These are just questions you should ask, and let the DM help you answer, by crafting a story together.

Hopefully this is a much more exciting part of your backstory to create, because you can ask the most ridiculous question, and who knows what answers you are going to uncover?
If you can give your DM a 1-2 page write up, that includes hopefully a good handful of each of these two topics, than you will find that your adventures are much more engaging than just “Find and Fight, Reward and Rest.”