Getting started on your work is not always an easy thing to do. It's a prospect that seems to scare some people. One thing to remember however, is that writing a large work must be done like eating an elephant, you have to do it one bite at a time. Writing a novel is a big project, and like any big project, it’s easy to procrastinate about getting started.
One thing that makes this easier for me is the fact that I don't write from beginning to end. I jump around all over the place, so when I get a good idea, I just start writing. It doesn't matter if the scene I'm thinking of happens in the middle of the book or the end, I just start writing and then come back in and fill in the gaps later. I honestly don't know how many other writers work this way, but it’s been the best method I've found that works for me.
The main point that I'm getting at here is if you have a good idea, get it on paper or into your computer as soon as you can. You might be pleasantly surprised at the way words flow once you convince yourself to simply get started.