The Girl of Steel is a silver age comic book story which appeared in Superman #123. It features the first appearance of a girl character with superpowers like Superman's. The first character to be called Supergirl was a teenager girl called Lucy of Borgonia who appeared in Superboy #40, but she had no powers.
Jimmy Olsen discovers the ability to have his wishes come true. His first wish: for Superman to have a super-powered female friend, who would help him in his fight against crime. The product of his wish, is Super-Girl.
However, upon discovering that such a being is a bad idea, Jimmy wishes her out of existence.