So how do you not know what your story is going to be about, and still tell a story in the first half of the novel? I’m not suggesting just starting blind (though if that’s your thing, do that) or suggesting you plan a lot. But there is a middle ground where you dip your toe into the water of this new world.
There are lots of guides out there to walk you through how to plan your plot. What conflicts, or tension, or backstory or whatever, but this takes a step back from that. I don’t think preplanning what’s going to happen in your novel is the most important thing you need to figure out. I get my character first, then a few glimpses of future scenes that may or may not make it to the final version, and what the character wants, initially. But I also think that stories come in two parts, the story you want to tell, and why the story you want to tell is going to be different from all the other stories that follow the exact same path. No matter what path you’re going to start out on, it’s going to be a well marked at least in the beginning before you get into too many branched decisions off from the beginning.
I think, for me at least, the two pieces of information I need to know is what is the story I’m trying to tell going to be about, and how I plan on making that plan different from all the other stories written about the exact same thing. The most important part of that is the second part. While you can write until you figure out what the thing that makes your story different, but then you have to have the courage to cut all that comes before it. If you start out your story on the right hook that will show the reader that this X is different from all the other X’s out there, your story starts where it should, even if you have to redo it in the rewrite.
I’m not one to say planning is better than pantsing when they both have their drawbacks. A planned novel that has the characters sticking to the plan no matter what runs into the danger of having characters start acting out of character needlessly or doing stupid things that progress the plot because that was the best the author could think of before they started stomping around in the characters boots.
Pantsing, on the other hand, if not rewritten enough, can seem like plot points come out smelling of the author’s ass from whence it came. Nothing has any order and the episodic nature of the story tanks in the middle as the author does nothing to progress the tension from the cool idea to the neat ending, despite the 50k in the middle where shit needs to still happen.
There’s more, if you’re writing filthy commercial fiction, you should have an antagonist that has all the ability he needs to win, which means that you should start your book with the fail state in mind. And there’s more about traveling down from the infinite tree to the path of your finite plot. And then there’s the concrete how to front load your story with awesome, but that will have to wait.