Why write this?
 
It is important not only to have something to say, but to understand why you need to say this. You will spend a lot of time with your chosen subject, so be sure it is the right subject for you, at this time. You don't need to know the entirety of your story at this point--in fact it would be surprising if you did. But the decision to write about this must be firmly resolved, even if you still have much to learn about your topic. Once you commit to your idea, mentally resolve to see it through. This is important. Writing a book requires staying power, and there will come times when your resolve may falter.
 
Rules are rules, usually
 
In writing, rules rule. These rules are all about grammar, usage, syntax, and the other structural underpinnings of the written word. Learn the rules. It's okay to break writing rules occasionally, as long as you understand the rule you are breaking, and are breaking it for a reason that advances your writing.
 
Grammar books don't always help in the real world, so to see how certain grammar rules are applied you must consult the experts. Works by well-known authors should be punctuated correctly, and can be an excellent guide as to how certain complex grammar constructs are handled. Regularly published authors, with their legions of editors, are an excellent reference.
 
For example, how are quotes handled within an interrupted dialogue that also incorporates unspoken thoughts?
 
Sometimes you will find contradictory models in different books, or even within the same book. If the usage is ambiguous, it just shows that others are facing the same challenges. Keep looking!
 
Bottom line--learn the rules, use the rules correctly so that your writing will be grammatically correct, and when the rules are unclear to you, see how other writers handle the situation. Break the rules when necessary, but...
 
Know the rules. 

Outlining

Every writer has their own approach to outlining. I outline from the very beginning, literally creating a story line on a piece of paper before I write even one paragraph. I pencil in major and minor events at various points in my story, and gradually fill in the gaps as my mind conjures up new stuff. Only when I have a fairly complete progression do I actually start to write.
This helps me in a couple ways. First, if I have a general idea of how the story will progress, I can build the foundation for events that I know will come later. Second, this outline gives me something to write towards. If my character is on Hill A, and by the beginning of the next chapter he has to rescue a maiden on Hill B, then my task is to move the character from A to B. It helps to know where you're headed, and the outline is a critical part of this process.
Sometimes writers outline only when their work is finished, as documentation. Everyone works differently, and for some this may be fine. In my opinion, if you're going to use an outline, you might as well do it when it would be useful as a tool while your work is in progress.
 

Rewrite this

Just accept it. Your writing will never be as good as you can make it. One more rewrite. One more rewrite. After twenty rewrites, you'll still be saying the same thing. That's fine. You have to rewrite. When I start to get near the end of a manuscript, I try to find the weakest chapter, the most flawed page. Then I endeavor to make this weakest chapter into the strongest chapter, this flawed page into the best page.
 
Often it comes down to flow, the turn of a particular phrase, the choice of a different word. This is what rewriting is all about--deciding if there's a better way to say it, and then saying it. Rewriting is a perpetual cycle of finding a better way. It never ends, but at some point you must stop. When do you stop rewriting? I can't say.
 
Other tidbits
 
Make backups of your work. Even go so far as to periodically print out a full copy of your manuscript. Computers crash, hard drives get wiped out, CDs fail. I've worked with computers for many years.
 
Back up.
 
Every writer needs time away from writing to rejuvenate and reenergize. Make time for these breaks. Make time for a good night's sleep. Eat right. Exercise.
 
What is your book about?
 
This is the most common question potential readers will ask you, and you must be able to answer in one coherent sentence of less than four hundred words.
 
When someone asks about your book, you need to hook them. A dissertation won't accomplish this. You need a good one-liner to sum up your story and capture interest.
 
"My book is about..."
 
This summary will be used over and over for media quotes, interviews, and anywhere that you are in a position to discuss your book. You will need this sentence, so perfect it.
 
Don’t give up
 
Finally, don't give up. Although you will get discouraged, and you will realize, time and again, that everything you have written is garbage, and you will decide you are a fool to think you had any talent as a writer, be of good cheer. You are on the right path.
 
Any writer that has never had these thoughts is either a liar or will have these thoughts eventually. Self doubt is part of the game, maybe because writing is such a personal revelation. It is no great failure to not be a good writer. Good writers are in the minority, and vastly so.
 
What is a failure is to be a good writer, and to not use that talent. Allow yourself the occasional discouragement, but don't tarry in that land too long. Write yourself out of it.