I agree with the others that you should start off small (I should probably follow my own advice, but I have experience with writing so I think I'm good). Also, if you're going to use a very common plot device or overused mainstream genre, change something about it that will make it more "unique".
Example: I like to write a lot of fanfiction, and when I started on my first Dragon Age Inquisition fanfic I wanted to use the "modern girl in Thedas" genre (which wasn't common), but I change how she got to the DA world different than other fics (a way my reader didn't expect so it drew them in and got them engaged in the story).
Also, make the characters consistent while have a reasonable character arc. Having characters that will not only help shape the world the story is set in but also grow and change throughout the story will also help keep people interested in the character and may even have your readers rooting for your character to make it to their destination (another example from my fanfic and with my original character, who my readers seem to love her to death, flaws and all).
Sometimes it might also help to start with a template of some sort, whether that is using other movies, video games, or comics as the basis. For me with my comics I use inspiration from multiple comics to not only put the world together, but to also plan out how the story will go (pretty much, I find something interesting from a popular series set in a popular genre and exploit it in a way that I can create an interesting chapter or story arc).
Don't know if this helps but this is my two cents.