Billy traditionally lives at an orphanage, he is anywere from 12 to 14 years old. (Probably 14.)
He has a twin sister, but he doesn't know about her yet.
He is friends with Kamala Khan, a Muslim girl, who he has a crush on, but only sees him as a friend.
How Billy gets his powers is up to you. Either:
1) A wizard gives it to him.
2) He exchanges places with Carol from another dimension
3) An accident awakes a latent mutation.
4) He gets the powers from being caught in an explosion with an alien.
(All these are Captain Marvel origin stories).