Writing a fantasy novel:

Step 1

The first thing you need to write a fantasy novel (besides writing implements) is an idea.
Too obvious you say, well it is obvious, but still correct and harder than it sounds. For you must find an idea that is: 1- workable; 2- not stupid; 3- interesting.

1- Workable:
This means your idea must fit into some logical structure; in other words your plot will have to have rules.
If you are going to write about magic, then you will have to put limitations and structure on your magic.
If you plan to break the laws of physics, you must be willing to explain why and how.
This also applies to supernatural plotlines; you have to define the whys and hows of such creatures as vampires, werewolves, etc.

2-Not Stupid:
Fairly self-explanatory. If you have an idea such as a magical bird that grants wishes, but all your characters wish for are crackers to feed the bird, it isn't a good idea. If they wish for world peace, only to have it threaten to end the human  race, then that is a good idea.

3-Interesting:
The idea must have merit to readers and also to you as a writer. If you lose interest in writing your book it will gather dust on a shelf, and if you want to sell books, people must want to read your novel.
Your book's subject must be something that inspires your writing passion. If you believe in the idea, then it can be unfolded into an interesting novel.

That is the end of Step 1.
In Step 2, I will show you how to take those ideas and start putting them on paper.