There are folks who think they've "got an idea" for a novel, but somehow they never get around to writing it. The thing is, "having an idea" isn't enough. That's a static thing. Unless you develop it, it's just "an idea". You've got to work with it, play with it and twist it around in your mind to bring it to life. That's how it grows: simply by noodling with it.
The solution is simple: make a place for your story in your mind, and visit it every day. You don't have to spend a set amount of time thinking about it daily, but promise to visit the idea every day no matter what. Stay with it for as long as it seems interesting, and then come back the next day. Each time, knock it around a bit and see what happens. Stretch it this way and that. See what other ideas sprout up around it. This is how a book happens.
And don't give up. For whatever reason, your idea might seem a bit stale today, but when you look at that very same idea tomorrow, you may find it gleams a bit. You approach it with a fresh perspective each day, and this will cause you to see new things. All you have to do is spend time with it. Think the idea through.
That's actually the first stage of novel-writing: thinking. I write on a calendar the day I start thinking about each book, and that is the start date for that book. I may not write one word for weeks, but the book is underway. It's alive in my mind.
During this period, don't go anywhere without paper and a pen. Jot down any ideas that occur to you, and keep them in a safe place. Thinking through the characters, the setting and the story is the first stage. Taking notes about your thoughts is the second. And then it takes off.
You move next to scene creation, writing down ideas as they occur to you. You picture your characters in a setting, and you keep in mind what the scene must accomplish. Every scene has to have a clear purpose or it doesn't belong in the book. Write it all down: how the scene looks, smells and feels. I find writing down snatches of dialogue is very helpful, because the story is already moving when someone is speaking. It's up and alive. Write it all down as it occurs to you. You'll find these notes very helpful.
The next step is the big one. You'll know when the time is right. Read your notes now and then, and improve on them. One day I suspect you'll realize you've got the first scene. It's just sitting there in your mind, waiting to be written. As soon as you can see it, even just the beginning of it, go to the computer (or notebook) and start writing.
And once you start, keep writing. Don't stop, just let it all out. Don't worry about it as you're doing it. JUST WRITE. Remember, you have nothing to lose. What's the worst that can happen? You'll screw it up? Big deal. What have you lost but a bit of your time? And you can always try the scene again if you're unhappy. Seriously. Say it after me: nothing to lose.
When you feel the magic, and you will if you've followed these steps, go for it. Get something down. And never stop while you're writing (and certainly never go back to change things to make them sound better). Let it go to the end. Just keep on going for as long as you can. Don't observe yourself writing; just write. You can always go back later and edit it as many times as you want. So for now, just keep going.
Think. Take notes. Visualize scenes. And when you can see a scene clearly in your mind, reach for the keyboard. You'll be glad you did.