I need some help making a backstory, I'm stuck

So basically as the title ensues I've hit writers block and I need some motivation or tips.

Basically what I have so far is that my whole character already who is a male Paladin that I'm building towards an oath of vengeance. He's a variant human and weilds a shield and spear with a glaive on his back. He has chainmail on and stuff.

I kinda want his backstory revolved around being born and raised in a church. Something like that, or maybe something happened to him that mad him into a paladin. Which could possibly lead to the oath of vengeance.

What do ya'll think?