You have an idea for your story, now what? It's time to figure out who will be your main character (usually the protagonist). Your reader needs to feel something for your character, but how do you create feeling for a character that is flat? Think of them as real characters (My characters are screaming at me, "We are real!!!") Assign them attributes that you would see in others. Some kind of quirk makes them more accessable.
Here is a character sheet that my niece and her friend developed when they were in middle grade or high school. They encompassed quite a bit in there character bio, it may help you to get to know your character better.
Some authors have their characters write about themselves or the author interviews the characters. Don't forget that you protagonist has to elicit emotions from your readers, they must be interesting. In "To Walk the Night" by E.S. Moore, Kat Redding isn't always likeable, she can be downright rude, but she is interesting. She is a highly emotional, usually angry, vampire, who happens to kill other vampires to keep humans safe. This is a simplified version of her personality, but it is why I like her.
You will, also, need an antagonist. This is the person or situation that causes your protagonist grief. Without conflict their is no story.