Posting is publishing, and publishing establishes a copyright (others don't copyright your story, they violate your copyright by copying the story and publishing it), without your having to do anything else. You could just put © 2018, Your Name at the end - your real name, not "Your Name". (That's hold the alt key and type 0169 on the number pad.)
But a copyright doesn't prevent people from copying it, it just allows you to sue them for damages for violating your copyright. (And you have to prove how much the damages are - for instance, if they include the story in a book they're publishing. Just putting it on a website doesn't give you anything to prove damages, you can just send a "take-down letter" (google it) to the hosting company asking them to delete the page (which they will). But you have to be aware of it. The only way to actually prevent them from copying it would be something like putting a blank picture over it. You can still read the words, but if you copy you get the picture - which is completely blank.