Deciding on a statement you want your book to make can help you power through the days when words are out of reach. Choose something simple like "life goes on" or something more complex like, "Watch the stars and see yourself running with them. (Marcus Aurelius)" when you are writing (and rewriting) scenes, use your statement as a test -- does it resonates in the scene? If not, there's probably work to do.
Whether you are writing a book, chasing a dream, or just trying to eat healthier, you have to start by writing things down. Get the thoughts out of your head and on to paper (or a screen) so that you can start to massage and shape them.
Whatever thoughts you have rolling around in your head will make more sense if you write/type them out. You won't get it right the first time, and that is OK. That is why there are erasers and back buttons. That said, as long as the words stay in your head, you won't make forward progress.
So grab a pen and write! Or type your goals into the comments. I can't wait to see what you have in mind.