Now that we realize that writers have to sit down and write, what else can be said about being a professional writer? A certain amount of logic is required to answer this question. Once you begin your novel, once you start to craft your characters and plot, you have to finish your tale.
This is much harder than it sounds. Every writer hits road blocks. Sometimes writer's block will just sneak up on you. But you have to keep going, taking your work through to completion. It is no lie to say that fully half of everyone who starts a novel will never finish one.
If it helps, try to write without caring too much about grammar, spelling, or getting your descriptions just write. Remember, you can tinker later. For the moment, just get your story out of your mind and onto paper.