There's a lot I could say here, but simple dedication is the biggest obstacle I see for many. I have so many friends who want to be writers, but in the end, distraction always gets in the way, whether it be the TV, video games, sports, Internet, or what have you.
If you're a writer who has difficulty focusing, I recommend first starting small--perhaps focus on a short story or two. Not only will this provide some good practice, but the sense of accomplishment received upon their completion will only encourage you to tackle that next story.
Just remember: Once the novel is done, make sure you have an objective source evaluate it! It might be painful to do, but you don't want to begin your career with a book everyone hates. Remember, revision is your friend! :-)