One group I played with some years ago, back in 2nd edition, had, as part of their character generation/background process, an explanation of what event caused that character to gain the experience points that pushed them from 0th level to 1st level.
For example the Ranger I played in that campaign was a country farmhand who grabbed an axe and successfully killed a couple of desperate goblin raiders who came to plunder the fields.
I really liked this idea, and am mulling using it again in a 5e campaign I've got planned for the autumn. But I wondered whether there was any official rules material to give rigour to this process.
So: is there anything in 5e rules and supplements that describes the process by which an ordinary 0th level person becomes a 1st level anything?
(This was inspired by this question, which I thought was asking the same thing)
Best Answer
There is no 0th level
D&D 5e expects players to start at level 1. NPCs don't have player class levels, so there's never such a thing as a 0th level character. A NPC fighter doesn't actually have fighter levels, he just has a stat block that incorparates some fighter skills into it.
Because there is no such thing as a 0th level character, there are no rules on how characters gained their first level, because that's how they started the game. How you wish to fluff your character becoming a ranger however is up to you. It might be related to the background you've picked, but it might not be. "I used to be a merchant, but now I'm a fighter because I defended my town from goblins" is a perfectly valid explanation. The rules don't care either way, they just care that you've made a level 1 character.