Here's how I see it.
A lot of your backstory won't make a lot of sense until the game gets started anyhow as we'll have to adjust depending on world and purpose of the campaign.
Here's what I want from your character backstory.
1. name
2. race and subrace if applicable
3. age
4. basic physical characterstics (fat, thin, burly, lithe, weatherbeaten, cheerful, stuff like that) it shouldn't take more than a few word. On the physical stuff, if you spend more than a couple short sentences on it, you've done too much.
background: in terms of actual background you choose.
class and subclass: I know many classes don't have their subclass at level 2 but I want you to make the choice at character creation anyway. In the case of warlocks, who effectively choose their subclass twice (once at level 3 and once when they pick what kind of patron they have) all I need is the patron.
traits: the 2 traits you pick as your character traits from your background, you are free to mix and match them up and aren't limited to what the book provides, and you can make up or change them as you want provided they still make a coherent character. Also explain your bond, ideal, and flaw. You must have one of each of the last 3.
alignment: what is your character's alignment? any alignment is okay except for chaotic evil.
a short written description talking about the backstory of your character, a paragraph or two is all I require but the more you give me, the more easily I can work your character into the world. That said, as we don't even quite know what world or type of campaign we are doing, keep things relatively vague.
if you choose things that are campaign-specific such as being A waterdevian noble (from Sword Coast Adventurer's guide and being specific to the forgotten realms.) and we don't play there, you can either say you are from that world (I allow a lot of hopping about the multiverse) or I can create something close enough to waterdeep to satisfy your background. Basically if you are a Waterdevian Noble in a world where Waterdeep doesn't exist, I will create a flourishing powerful city-state with some of the key aspects of waterdeep.