I'm trying to construct a D&D 5e dragonborn paladin character for my first game, and what I'm struggling to work into my character's backstory is: how did he become a paladin? Like, how does anyone become a paladin? By training under a knight? Working for a king? Demonstrating exceptional valour, like a non-posthumous VC award or something? Simply being a stand-up awesome guy?
He's also a Chaotic Good character, which fits in his character - outcast, strong guiding conscience, but not too concerned about how he fights evil - but not necessarily within the ideal of a paladin. So if there is some "order" to which all paladins belong, he probably left it a while ago, due to not fitting the mold.
So, I'm just looking to understand the most commonly accepted lore or mythologies, or even world histories, for how paladins come about.