Well, I play a Cleric. So I took that and expanded off the fact I knew I wanted to eventually become a Contemplative, which, according to the fluff of the class, means you have to have had actual contact from your deity. I also knew I needed a reason for traveling, and our DM likes us to have a "nemesis" of sorts to be a possible random encounter or side story. So after I had all the pieces in front of me, all I had to do was expand.
I did this, My Cleric was forced away of his monastery after he assaulted a higher ranking official of the church with a mace after he discovered this man was forcing himself on the female citizens of the local town by telling them he was going to bring them in for worshiping Evil gods or practicing evil magic. Even though my character was morally justified, assaulting another member is expressly forbidden, especially someone of a higher position, which is usually associated with having a closer relationship with your Deity. He wasn't forced to leave the church, but he was asked to take requests away from the church. When he left, he was told by his god that he was still in his favor, and would still be able to perform his duties as a holy man.
All you have to do is lay out what you have, think about what makes you really want to play that class or race, then add a little personality to it.