The best way to prevent anyone from stealing your idea, is to have a rubbish idea, and write it badly.
Anything more than that and what you write will become part of the canon and people will build from it.
You're protected from someone stealing the details of your story, but that isn't generally how work is copied.
I remember when I first saw Iron man, thinking that I'd really like to see a story that was based on Pepper Potts. with all of the other mayhem going on in the background.
In that context I may have an issue calling her Pepper Potts, and the superhero couldn't be Iron man, but I could write something that built on that story by changing the names to protect the guilty (i.e. me)
Romeo and Juliet, is probably one of the most known stories in the world, it has been copied, rehashed, built on, stolen from. Imagine what we would have lost from the world if Shakespeare had been worried about someone stealing his ideas.
Stop worrying, get writing. If its good, people will steal ideas from it, take it as a compliment, and move on to the next thing they can steal.
We're here to be creative, we take ideas from the canon, build on them, and give something new back for others to build from. If you don't like that thought, go be a bricklayer.