Writing a book is incredibly difficult. Writing a great book is practically impossible. When you sit down to write yours, ask and answer three questions. If you break writing down to its simplest form, you'll find it's really not so difficult after all. Master the basics, and all the rest is just polish.
Every novel, no matter how thick or complicated, revolves around three specific questions. Ask them, and make sure you know the answers, when you're writing yours.
Every novel needs at least one main character. Juggling more than one main is hard, but it can create a very rich and engaging story. Make your main character(s) interesting and identifiable, and your readers will enjoy finding out about them.
Every book has a setting. Research yours to make it real and rich on the page. Readers want details. What's the weather like? What are the buildings like? What do the rooms look like? Good descriptive writing paints a picture without taking over the entire book -- remember that no one wants to read your rambles about the way the curtains hang. Strike a good balance, and use the detail to add to the story instead of allowing it to swamp the story.
You don't have a book if you don't have a plot. Stuff needs to happen in your book. Allow the readers to get to know the characters through specific events. Readers want to be put inside the story; they don't want a story told to them. Use plot to make your book happy, funny, exciting, sad -- any emotion you want to evoke.
If you can answer three questions, you've got what you need to start writing a book. It's the idea and the imagination that matters. Mechanics will come later, after lots of editing and hard work. Once you've found your three answers, the really hard part is already over.