The basic idea is to write an interesting story, of course, but also to read and comment on other stories. By leaving other people helpful feedback, they might be more willing to return the favor.
I see you're writing a 1D fanfic, so definitely make sure to include one direction in the tags. It'll make it easier to find for people looking for a one direction fanfic. What else? Time, you should really just try to have regular uploads and be patient. It only takes a few people who really like your story for word to spread.
You can promote your story in the share your story club: http://www.wattpad.com/club/38-share-your-story and outside Wattpad (Twitter, Facebook, etc...)
I hope this helps you a little. Good luck with your writing :)
~Vicky