Create your settings and people first. Fantasy books tend to be based around completely made-up worlds and cultures. You'll need to have everything in place for your settings. This may include writing a language, or at least hinting at a language, for your people. Create a background world to set your story in and write a historical background. It will be a great deal of work, but if you want to make your new world realistic, these are things you'll need to know.
Introduce the quest early on. At the heart of every fantasy book is a quest for something. It could be the quest for an object. In all probability, your characters, or at least your main protagonist, will be on a quest to save the world from some evil force. This element of the story needs to be set up and brought into play quickly.
Introduce conflict early and continue finding new ways to stop the quest. Never allow your protagonist to achieve the quest without working for it. In fantasy, conflict comes in the form of dragons and wizards and other similar elements that give life to the fantasy story. Make sure you populate your story with plenty of these elements and introduce them to your readers right away.
Engage the reader's imagination from the start. Use descriptive writing to bring readers into your new world. Describe the world, the people and the quest in such a way that your readers feel as if they are a part of it. Fantasy novels are often huge in scope and present the writer with plenty of opportunity to sweep readers into completely new and exciting worlds full of fantastic adventure.