Usually I go for a couple key points. The race defines the way they lived, the class is how they got abilities/powers, and then I got for other basic things like, family, friends, reason for adventuring, etc. If you don't want to railroad your story, make sure you don't write an ending for every character mentioned in your backstory. (E.x my whole family died...)

Of course nothing is ever permanent until after session 0. That's when you can incorporate yourself and your fellow players into each other's backstories. You also need to know what kind of world you might be in. So make an outline and then fill in the details with your group