Creating a backstory isn't that easy. Here's a good example of one.

(Not a real character)
Dawn used to be just a plain animal. But before her father passed away, he gave her a special necklace that makes her what she always wanted to be. A spirit animal. But later her friends get poisoned. Dawn didn't know, but the poison made her friends get rid of her without dawn knowing. Dawn doesn't know what's going on, but she wants revenge. She was so mad that her necklace got poisoned to make her evil. She wants revenge on her friends, when it's not there fault.

Basically the backstory is what tells about what's the main idea of the book. Here's one that's not a good impression.

(Again, not a real character.)
Snowflake is nice and caring and a good leader and likes to make friends and wished to be queen of her kingdom. Everyday she gets up in the morning and pretends she's queen. On Tuesday she got up and pretended she was queen. On Wednesday she repeated. She likes being queen or pretending to be.

See the difference?? So how to make one you MUST need these first before you do anything else.

1. The character's personality.
2. The problem the character needs to face.
3. What the character likes.
4. Your creativity, DUH.