You hear people say, "I'm going to write that book someday." But no book ever appears. They honestly want to write the book but somehow don't get around to it.
This seems like a tragedy to me. If you have the urge to write -- and especially if you have the talent (or think you do) -- you should do it. What have you got to lose? I say write that book or story or play right now. It's simple: just begin.
Think about it. You could close this browser right now and open your word processing program and start that book. What's to stop you? You have an idea in the back of your mind; you know you do. It's been there forever. So why not take your idea out for a test drive? Give it a shot: write a part of your book, any part at all. It all begins with a sentence and you know you can do that.
If you never take a chance, if you never write that first line, your idea will remain just an idea. I'm serious when I suggest that you start your book right now. Just think of an angle to approach the story and visualize the first scene that occurs to you. You don't have to know everything about the scene. If you can see its beginning, go for it. Write that first sentence and once you've put a few words down, keep going. Write as much as you can without stopping.
Just think -- no matter how the writing session goes, when you're done you'll have something to look over, to consider. And you know what? You're already on your way. That's a bit of your book that you're holding in your hand (supposing you printed it). Already your idea has entered the real world. It exists. And if it went well, you may have just written your first scene. (And if you didn't, don't worry. Your writing will improve with practice.)
Sure, you may toss it away in the end, but so what? Once you've written a scene (or even part of one) and stepped into your fictional world, you'll find it soon becomes familiar. You'll feel more comfortable the next time you return to it because it won't be alien territory. You've been there and have something to build on.
Of course, you'll need to think through your book at some point. I don't mean to minimize the work you'll have to put into into it. Everything won't happen in a flash the way this scene will (if you take my advice and try it now). I suggest this method only to jar your mind into seeing your book as a real possibility in your life. If you take my suggestion you may end up saying, "I really can do this. I can write this book!"
That's all it takes: that first step and right away you're doing it. You're writing and it's not just a dream anymore. If you've been fearful about starting your novel, getting through the first day of writing will help to drive away the willies. And I think you'll find that as your fear diminishes, your view of your book will change. You will become more comfortable with your fictional world, and more willing to return to it.
The transition from fearful non-writer to excited writer can happen in a flash. All you have to do is take that first step. I'll say it again: what do you have to lose?