The fantasy genre is huge and varied. So first you should figure out which sub-genre you will choose. There’s High Fantasy, with speech and word patterns that somewhat resemble old English and usually involve royalty and royal magic. There’s Urban Fantasy that’s contemporary, though it may or may not take place in a city. There’s Light Fantasy, which is a book that falls in another genre but includes some fantasy elements. And so on and on and on.
Once your sub-genre is chosen, read a half dozen or so books in that sub-genre and jot down three to five things that all of the stories contain because those are things that readers of that particular sub-genre have come to expect and will want to see in your story.
If the fantasy world is other than our normal, real world, spend some time building that other world. But don’t spend too much time on this or you may never get around to writing the story. Instead, do as much as is needed to get the story sketched out, then keep notes on things of that world as you write so you keep everything straight
Plot out your story and write it. And good luck to you.