It is better to write a bad first draft than to write no first draft at all. ~Will Shetterly
We've all heard it said "You can't fix a blank page." It seems almost like one of those things that isn't necessary to say out loud, yanno---because, 'DUH!' What can I say? In order to finish, we must first begin. And we can't finish if we're tied up in knots that what we're writing is bad. Get the idea down, then worry about refining it when you're done. Try not to let worry and doubt stifle creativity. Yeah, easier said than done. But what do you have to lose by trying versus what you stand to lose by NOT trying?
By writing much, one learns to write well. ~Robert Southey
A variation on "practice makes perfect." Self-explanatory. I've noticed a BIG difference in what I've written recently compared to stuff I wrote years ago. You just can't help but get better as you continue writing, and by being open to learning as you go. This is something I find myself returning to often---by being teachable, willing to learn, I can only become a better writer.
You never learn how to write a novel. You just learn how to write the novel that you're writing. ~Gene Wolfe
Every novel is different, for sure and for certain. Some just roll right out of the fingertips, others require blood, sweat and tears. Doesn't mean one is more worthwhile than the other. Doesn't mean you should abandon a difficult WIP---you just need to keep in mind it's like a child---no two are the same temperamentally or personality wise and shouldn't be treated like they are.
And because I'm a fan of learning, here are a few links to go along with my interpretations of the above quotes.
Writing the First Draft ~ Netplaces: With tips on 7 issues a writer might face in regards to writing the first draft.
Creative Writing for Dummies: No, I'm not calling anyone a dummy. I thought this was a pretty good bullet point list for getting started.
Valerie Parv's Muse Obedience School: What can I say? Valerie is a very dear, personal friend, and she speaks gold. On top of that, she writes gold, too.
Ten Reasons You Should Use Writing Prompts ~ Deanna Mascle: One of the reasons? "Practice makes perfect." 'Nuff said.
On "How to Write"---so many choices. You can google "How to write a romance novel" to "How to write science fiction"---you'll get all sorts of sources. These links for for the general "How to write a novel."
How to Write (and Finish) A Novel: A list of about 5 items for writing in general.
How to Write a Novel ~ Simon Haynes: More detailed than the first link with suggestions on characters, plotting, research.
Learn the Elements of a Novel: A site that lists building blocks and links.