There are four stages of starting anything. I'd tack "for me" onto the end of that, but since I'm writing this, I'm gonna go ahead and assume you've worked that part out already.
Step 1. Boredom
This is how the whole story telling thing began for me - long stretches of silence with nothing to do while tucked into the backseat of a car while the family drove from one military posting to another. Long stretches of highway, watching the scenery go by, imaging what went on in the forests or in the towns behind the facades of houses and businesses. Eventually, I liked the stories playing in my head enough to write them down. Once that happened, I could be bored anywhere and come up with a story idea. Usually as snippets of dialog. Angsty, drama-ridden dialog, but you have to start somewhere, right?
Step 2. Names and Situation
This is my proof of concept step. From angst and drama, I have to come up with a compelling situation - the background, the world, the people, and a general sense of how the story might begin and how it might end. Ish. I need only a general notion. This, for me, is no time for detail. Lots of note taking and journaling happens at this step. I play a lot of 'what if' games. But one thing is certain, without character names, I'm dead in the water. The protagonists must step forward and identify themselves. This is where I need characters to develop the beginnings of voice - this is the illusion that these people actually live and breathe and have wills of their own outside of my imagination. This is the point that the monster has to rise from my laboratory table and either go forth to wreak havoc or collapse in a mass of stitched together body parts that are all going to have to be burned before they start stinking up the place.
Step 3. Character, Character, Character
Once I have a general notion of a story and some character names, it's time to dive deep, and for me, everything comes from character. Everything. The plot, the conflict, black moment, and the climax. I spend about a week working my way through Mary Buckham's Break Into Fiction templates for all of my major characters. Protagonist(s) and antagonist at the very least. If it's a romance, I'll work through hero and heroine. This step also serves as my initial immersion point for the story - meaning that at this stage, I'm spending several hours a day buried in questions about who my characters are, why they are who they are and what they believe they know about themselves but have totally wrong. This is where conflict is born for my stories. It's also where scene lists begin building. If you're writing genre fiction, you have to build scenes that challenge your protagonists' assumptions about themselves and motivate them into change (there's your character arc). If you're writing literary fiction, your scenes will rub a character's nose in his or her faulty assumptions, but not force the character to change, though he or she may come to comprehend his or her faults.
Step 4. Word Count
This is the point at which there are no more excuses. Armed with a few markers (I usually know how a story opens and I have a soft grip on how it might end - everything else is a blur) and with a pretty good understanding of my characters and what drives them, I can start making tracks. The form of my story is still vague. I'm usually flying blind once I get past the first few opening scenes, but with my character's templates filled out, I have guideposts to keep me pursuing their goals and challenging their weaknesses even if I don't know exactly what happens in the middle of the book.
That's my summary of 'how to begin'. Do you follow any kind of pattern for starting something? What I'm curious about is how plot-driven writers approach starting a book. (As opposed to a character-driven writer.)