Many writers never venture into the world of novel writing because of the daunting proposition of taking that incredibly difficult first step, and therefore their great idea remains just that: an idea. But writing a novel does not have to be as difficult as it seems, and many people find once they start the novel, the rest comes out a little easier with a combination of creativity and determination. Certainly, writing a novel is a difficult process, but it becomes much easier once a writer takes that extremely crucial first step.
There are two general schools of thought concerning writing a novel: many authors will meticulously plan and outline the novel, while others will simply write the beginning and take a "see what happens" approach. One process or the other will work for you, but if you're having difficulty choosing, try this: write the ending of your novel first. That way, you know where you need to end up once you start writing the beginning. This process of writing a novel is akin to getting directions to a new restaurant before you leave the house. You know where you need to end up -- it's just a matter of getting there now.
Another method to getting started when writing a novel is to develop a main character that you care about. Don't worry about the plot right off the bat; concern yourself with writing a character who you want to follow around for three or four hundred pages, someone you wouldn't mind palling around with in real life. That way, you will be consistently interested in what happens to them throughout the course of your novel. Where do they live? What do they look like? What struggles are they dealing with in their everyday life? If you care about your main character when writing a novel, the plot will begin to develop itself.
Once you have gotten started writing a novel and you have a character you care about, it's time to throw in a complication. The easiest way to achieve this is to think of your book as a series of "What Ifs." What if this character met that character? What if my truck driver main character has an accident? What if my main characters get a divorce? Many of the best novels out there were begun on this premise, and if you're having trouble getting started, this might be a good approach for you.
When writing a novel, be sure of two things: first, be sure you care about your main character. This will make it easier for you to write what happens to them. Second, be sure to add a complication, or "What If" scenario. Keep asking questions and your characters will keep answering them.