Write a prologue (often called prelude in fantasy books) that sets up the world of your story. This prelude should introduce not only the types of characters that will be encountered in your story, but some of the conventions and laws of the world readers will need to know to understand the story. If there is a legend (there almost always is) that forms the basis of the story you're about to tell, the prelude is the place to tell the story.
Introduce the main characters of the story in the first chapter. Fantasy novels are typically quest novels, meaning something needs to be achieved. It may be finding a magic stone or sword. In fantasy novels, the quest is usually performed by a group of characters that work together. This working relationship is often strained, with a heroic character and a group of misfits working toward the same goal, often reluctantly. By the second or third chapter, begin bringing these characters together.
Write an inciting incident that makes it clear to your characters they will need to work together for the good of the people or the future of the world. An inciting incident is an event that sets up the rest of the story, and the quest in particular. The inciting incident should be of such magnitude that there is no doubt in the minds of your characters that they must work together.
Set your characters on their quest. This is the real start of the story. The quest is the journey your characters will take to bring the story to a conclusion. Everything you've written to this point sets the tone for this journey.