you really can't stop someone from "stealing your story". if i was a crazy person (we're speaking metaphorically here), i might spend a couple hours copying something from a computer if i seriously wanted to STEAL someone's story.
i read somewhere that there's nothing new under the sun. you might come across a story someday that you'll think shadowed your own and find out that it was written before you were born!!
so just post what you want, have fun with writing, and use your talent.
I think people are less likely than you'd expect to steal art. ^^ ~parameister is right about being able to prove it's yours, too. Go for it!
I don't watermark my art (not like it's steal-worthy anyways but) because I like to share it how it's supposed to look without being marred, and if someone steals it, well...It's too bad they have such confidence issues they feel like they need to steal art. I can always prove it's mine if I need to.
omg I know what you mean D: like I have so many story ideas I want to share but eeugh.. but really, if you post it now, it'll have a date on it so if anyone does copy or steal, you can prove it's your by showing people the date you posted it. and chances are, if they have to steal someone else ideas, they're probably not very good and won't get much attention. and if they do, I'm sure someone will notice that it wasn't theirs to begin with.