When you write something, you own it and, in the UK at least, the laws of copyright give you automatic protection against your work being stolen.
There’s no need to plaster your work with copyright symbols and declarations of ownership before you submit it: good editors and agents already know the rules of copyright, and don’t need to be reminded of them (and you’re not going to submit to anyone dodgy). Posting your work to yourself by registered mail is known as "poor man's copyright" and as far as I know, it's never been enough to protect anything, let alone be acceptable as evidence in litigation.
There is no copyright on ideas, only on the execution of them. For example, Bridget Jones's Diary was a re-working of the story of Pride and Prejudice, and so caused no problem with copyright infringements. Had Helen Fielding simply submitted Pride and Prejudice with Austen's name scribbled out and her own put in instead, then that would have been a copyright infringement (OK, so that's a bad example as Pride and Prejudice is well out of its copyright period, and strictly speaking such misuse would be plagiarism rather than copyright infringement--but you get my drift).
The upshot is that you can't protect your idea unless you actually write it. Then your specific arrangement of words and the various specific ways you've expressed your idea, such as your structure, detail and characterisation (which together constitute the story you've written) will be protected, but the idea (the storyline) will not be, as ideas are fair game.