(I think it'd make more sense to say 「たすけてください」 or 「たすけてくれませんか」 rather than 「たすけておねがいします」 btw)
I'm not entirely sure of the correct correct way of setting it out, but I've never been reprimanded for the way I set it out.
(Informal) Email: Start off with _____さん, start writing the body of the...