okay let me give you a run down
Fallout 3: You are 19 years old and your dad escapes from the vault you're in (a vault is a shelter from nuclear bombs), so you set out to look for him.
New Vegas: You are a courier who is shot in the head by a man in a suit while carrying a package to the New Vegas Strip, and you set out to find him.
Both worthwhile games, Fallout 1 + 2 i haven't played