Prerequisite: While there's nothing wrong with diving in headfirst to jot out that masterpiece, it could save you a lot of time and headache to begin by honestly assessing your general writing. Do you struggle with grammar? Do you lack basic knowledge of the flow of novels? Don't know the difference between omni tense and third person? No worries! If you feel you need help in areas like these, check out forums online:
Creative Writing Forums
Absolute Write Writing Forums
Or head to the library to find some books on writing:
Stephen King's On Writing
Anne Lamott's Bird by Bird
William Strunk and E.B. White's The Elements of Style
Read. Read, read, read. Read in the genre you're writing, read in genres you're not. Probably the best thing you can do to strengthening your own writing is to read observantly.
Got all that covered? Fabulous! Now, we'll move on to the writing!
Step One: The Spark.
Every story starts with an idea. It can be an image, an imaginary voice in your head (don't lie. You get them, too. right?), a personal experience, etc. Whatever it is, it's something that should grab your attention and keep it.
Step Two: The Outline (or The Pants)
As a person who thrives on following specific orders, I struggled most with finding my 'way' of writing, because there is no one way. Every writer tackles this step differently, though it can be broken down into two basic categories: Outlining or Pantsing.
Outlining: An approach where you start with your spark and brainstorm what-ifs until you can flesh out a point by point outline to follow as you write. Some use the snowflake method or just write out chapter by chapter how you're book will begin, weed through the problem, and end. Many people use writing programs like Scrivener to organize all their points into an easy to follow outline.
Pantser Style: It's writing by the seat of your pants. You don't know what will happen next, you're just going with the flow and seeing where your characters and story take you. Some writers swear that by forgoing an outline, they're able to craft more exciting stories by not limiting their possibilities.
Whichever route you choose, know that you can always change your mind! Maybe you start out pantsing, but reach a plateau and need to outline to sort things out. You might have a solid outline, but once you begin writing you realize it's not working out and you toss it aside--that's fine, too! Every writer, and even every book, is different.
Step Three: Rolling Up the Sleeves (or what writers like to call Butt In Chair Time)
Daydreaming about making the New York Times Bestseller list is great! But to get there, you have to, you know, write the book first. There will be days you write thousands of words. There will be days that you'd rather scoop out your eardrums with a rusty spoon than write one complete sentence. It's natural and it's normal to love your book one day, hate it the next. The key word here is perseverance. If you really want to write, you need to draw on your love for it to power through. Some folks find it helpful to set a word count goal for each day. Even if the writing is horrible and ugly, you keep going. You can fix the blemishes down the road.
Step Four: Beta Readers
We talk about them so much on this site for a reason. After reading your own book so many times, you become desensitized to it. You'll gloss over those misspelled words, misplaced commas, and even overlook major gaps of logic (because you know what happens next. The reader doesn't). That's why it's CRUCIAL to find some beta readers to read your book when you've finished your draft. Make sure you do your best to catch any mistakes, though. Don't hand them over something full of flaws because in your excitement, you couldn't wait and didn't bother reading through it with a critical eye of your own.
Step Five: Revisions
Armed with your beta reader notes, you'll start tackling those issues with your book. Some will be simple fixes while other issues can require a lot of work. If your characters are flat or the story arc fails to take off, you might be heading back to the drawing board and nitpicking your baby apart. This is good. This is necessary. This is rarely fun, but it's probably the most important part of the entire process so treat it as so.
Step Six: Write a Query and Send It Out
Those handy writing forums linked to above? They've got great places to learn what a query letter is and how to go about writing one. AW even has a thread specifically devoted to other writers critiquing your query for you (be sure to read some others while you're there and return the favor!) The query letter is your door to nabbing an agent so it needs to be the best it can be. And nerve wracking as it is, it's time to set that book free and see what happens.
Step Seven: Keep Writing
Didn't nab an agent with this book? Keep writing. Have several agents reading your work? Keep writing. Still waiting to hear back from agents? Keep writing. Notice the trend? A writer writes, and writes often. With everything you write, your skills will strengthen. Either your book won't make it to a shelf or it will, but regardless, you're going to need to write another one.
Okay. So obviously this isn't everything you need to know about writing--the truth is, writers never stop learning--but hopefully it's a great starting point for those who've had a few false starts but are ready to dive in!