The question pops up at three different stages of writing. Depending on the situation prompting it, it yields three different answers.
1. You have an idea for a writing project, and you sit down to give it a whirl. You can begin anywhere, anyway, anyhow. Whatever the project, the words you set down in your first session will not be the words that stand first when it is in finished form. Remember, writing is really about re-writing. So put your fingers around a pen or on that keyboard and just do it. You won’t know what you think until you see what you say. And once you have written something, you have something to play with.
2. You’re well into your project, if not finished with a rough draft, and you’re wondering what material to lead off with. It must hook your readers, pique their curiosity, propel them forward. You are thinking in terms of strategy now, zeroing in on tension. Stories begin when flow hits resistance, when a pattern gets disturbed, when differing needs meet head-on. So give such a moment prime position in a dramatic scene, and you will generate the big-bang energy to keep your fictional world expanding.
3. You have chosen the writing life: every day, you have to sit down and begin. Even if you’ve reached Chapter 20, you have to begin Chapter 21, confront new unknowns. This may be self-evident, but it’s not necessarily easy. Writers experience inertia, and it may increase the farther into a project they go.
When inertia keeps you from beginning, it’s usually rooted in self-doubt. It’s reinforced by a little voice in the head that says things like, I’m not a real writer. This story isn’t leading anywhere. It’s a waste of time. I should be doing something productive.
In a way, self-doubt and other forms of psychological discomfort are the side-effects of serious writing, which is so often reopens past wounds in order to transform them. So the first step to overcoming inertia—that conundrum of wanting to write yet not feeling like it—is to acknowledge the doubt and fear that’s holding you back, even free-writing about it, then taking the leap of answer #1: just do it.