Writing a book requires some kind of plan. Whether you're going to plot it out or start with an idea and let it develop as you write. If you're able, try to write a summary of your book. A synopsis. It doesn't have to be the "exact" thing that will develop in your book. Just what you think will happen now. When you're writing, things may change. Characters may do other things. You can change your synopsis.

Sometimes, people write chapter outlines. All of this will help you see if your story idea is developed enough to fulfill a book. These can change too, as you write and see in more detail how your book turns out.

One of the great ways to help our story idea develop, is to play the "what if" game. Start with your idea, then say "What if..." and ask yourself something like "what if I killed this character? What would happen next?" Then develop your story idea some more, then do the "what if" again. Each time putting your characters into dangerous or emotionally challenging situations. Give them obstacles to overcome. Goals to accomplish. A major character evolvement. Help them fight/resolve their outer conflict and their inner conflict.

Have you looked at the thread going on about Books on Writing? There are some really good books you can read that will help you determine if you're idea is ready for a story. I can't recommend "On Writing" by Stephen King enough. It's a great book.

I hope this helped.