You Decide
In the end, really only you can decide who your character is. After all, there are infinite ways to play a "Chosen One".
You said, "standard Chosen One stuff". What does that mean to you? How do you picture a standard chosen one?
As Shakespeare said, "some are born to greatness, some aspire to it, some have it thrust upon them." Were you born to a family of chosen ones? Do you want to be the chosen one? Are you forced by circumstances?
All the questions you asked are questions you should ask yourself. Keep asking yourself questions and your character will begin to emerge. Are you brave? Reluctant? Do you even know you're the chosen one? What was your family like? Are they alive/dead? Are you a war leader, a spiritual leader, something else? Do people accept you as the chosen one? When you visit a village do they all cheer you, or not even recognize you, or even laugh at you?
You don't even have to decide these things now. Perhaps your character is just an acolyte or a petty criminal or a soldier, and you the player know they are destined to be the chosen one, but they the character don't. Perhaps whatever the defines them as the chosen one hasn't happened. Or maybe in the back of their mind they just know they are meant for bigger things, but they haven't figured out how to get traction yet.
The "Backgrounds" feature may be useful to you
It sounds like you need a framework to help you decide what your character is like. Consider taking a look at Chapter 4 of the Basic Rules or the Players Handbook.
Some example backgrounds are suggested. You might consider picking one. For instance, the Folk Hero might be suitable for a "chosen one", although perhaps something else may be more fitting.
If you don't like the backgrounds available to you, you can customize a background.
The Folk Hero background suggests a "defining event". Looking at this may be helpful. If you don't like any of the defining events listed you can make up your own. Consider these two defining events:
- I broke into a tyrant’s castle and stole weapons to arm the people.
- A celestial, fey, or similar creature gave me a blessing or revealed my secret origin.
Either one of these might fit a "chosen one", but would probably be very different characters.
Similarly, the Personality Trait and Ideal tables might help you further define your character.
Talk to the DM and the other players
Generally you want to work with the DM at least a little to define your character. If the DM told you your character is "the chosen one", you might discuss in greater detail what they have in mind. Or if you have come up with the idea of the chosen one, you might want to work out with the DM what that means.
The other players may or may not go along with your chosen one. After all, anyone can say, "I'm the chosen one!" But someone else can say, "Yeah, and so's my donkey. Get your own dang beer."
Whatever you work out, it's your story, and it's part of the bigger story told by the DM and the other players.
