In fact, just about everyone who says he is waiting for inspiration to get started is really only procrastinating.
There's usually some kind of fear at work. Fear of failure, fear of rejection, maybe even fear of succcess.
If you want to achieve anything worthwhile, you must get started by taking action.
Action is the key to success in any endeavor.
That's how writing success happens. You have an idea, and you take action to turn that idea into an article or a book or a screenplay. You work every day, and you stay with it until you reach the end of the book or the project. Completing the project is your success.
By the way, selling a gazillion copies should never be the goal toward which you are working no control over that. Your only control lies with writing the best book you can write.
Even when you think what you're doing may be crap or a waste of time, you stay with it.
Persistence and consistency of taking action helps you achieve what you want.
Not only will you achieve your goal, but you will also have created the habit of taking action every day. That in itself is priceless.