Now that you have an idea, it’s time to start writing. My second piece of advice: Start writing as soon as possible for the longer you wait the more the idea will either be second guessed, lose its excitement or be replaced with a newer idea.
The scene you write first doesn’t have to be the start off the book, although having it be the beginning scene will make the story easier to write chronologically. What you do is extrapolate what you consider to be the best introduction of the idea and the best settings and characters you need to show it, and then figure out how they’re going to interact to this end.
I’ve found that the best way to do this is through imagery. What image do you think best introduces the beginning point of your idea? What is a good setting for this idea to begin in? How did the the main character get involved? Every story needs conflict so foreshadow or set off this conflict in the easiest way for your readers to understand.
In short, imagine a scene that best portrays the beginning of your idea to pass it off on the reader. By the end of the first chapter the reader will need a main character, a setting and the gist of what the story is about. Read more an example of this.
Step 1: The Idea
Portal fantasy (e.g. Narnia, Thomas Covenant, Princes of Amber) is reversed to become a space travelling science fiction story (e.g. Speaker for the Dead, Old Man’s War) where species from different planets interact through the same portal plot tool.
Extrapolate Beginning Scene from Your Idea
This is what you think will be the best scene to first introduce the main characters and plot tools that you’re going to be using for your story.
Step 2: The Scene
In a palace, a rabbit-like fantasy race celebrates victory over a lizard-like fantasy race with the help of a mysterious third race. One of the Rabbit-like people follows a man from the third race through a doorway, discovering that the third race is from a future earth where humans have been interfering with distant planets through superior technology.