The beginning is the hardest. As is everything leading up to it. You have to make the decision, whatever that may be, to commit. To say to yourself, "I'm going to do this," and really mean it.
And it's all downhill from there.
You'll remember the hundreds of other commitments (usually trivial in the extreme, like the washing up you've been neglecting all morning) you could be doing right now. Those suddenly look so much more attractive and exciting! They aren't, but the mind is adept at resistance and therefore can convince you of anything.
If you really care, you will see this for the lunacy that it is and step forward anyway. "Yeah, sure, doing that washing up will make my life marginally easier, but then what?" Reason is your friend - do not resist it!
So. You sit down in front of that blank sheet of paper. You are going to write. This is the beginning. But...you have so many ideas. All great and all fighting for your undivided attention! It's too much. Better to walk away now than to start something you will inevitably not finish. The fat lady is singing on this adventure already.
But if this were foolish then surely you would've brushed it aside the minute that it occured to you. The likihood of all those ideas being as great as each other is pretty slim. And even if they are, so what? Write them down, pick the one you want and file the rest away for later. Easy peasy.
As for finishing the damn thing...how the hell can you possibly know whether you will or will not finish if you haven't even started it yet?
Write. Begin. You'd be a fool not to.