I want to try to avoid repeating any advice you've already been given, so I'll focus on another area.
The thing that really separates fantasy (and science fiction, too, really) from any other genre is that so much of it will rely purely on your own imagination. Of course, any genre of fiction requires you to come up with a good story, and some interesting characters - but, once that's done, you can usually just get started and work out the details as you go. You might need to do some research on different topics to flesh things out, but you can worry about that when you need to.
With fantasy, though, you have to create the fictional world before you can write in it. If you're going for full Tolkien style high fantasy, then you'll need to come up with imaginary societies and cultures (and, everything that comes with that). But, even if you're setting it in the real world, there's still things you need to think about. Because, it's fantasy... it wont be the 'real world', will it?
If you want magic to exist in the world, then you need to give some thought to exactly how magic works. What are the rules? What are the limits? If you want to populate the world with fantasy creatures, you need to decide what exists and what doesn't. And, you need to decide how they work, too. Are vampires harmed by sunlight, for example? Or, do they (ugh!) sparkle? Are you going to base the story on a christian world-view, so that demons and angels actually exist? Is it going to be based on some other religion, or ancient mythology? If so, which one?
Obviously, you don't need to go overboard, though. You just need to create whatever is necessary for the story you want to tell. If you're writing a short-story about a haunted house, for example, you only need to make decisions about ghosts. Whether werewolves also exist in this fantasy world you're building doesn't matter for that story.