I would assume that if it's just coming out in English that it's about how Spawn came to be and following him from then on. Simple answer, but it's gotta start from the beginning.
Also, I don't know if this is being picky, but wouldn't that be a comic book as opposed to manga? Or are those the same thing, 'cause uh, I guess I can see the similarities, but manga consists of books, with chapters and such. Comic books, not so much.