In other words, how do you begin a novel if you don’t know how to start?
You start where the protagonist first engages with whatever problem the book deals with.
Not with the back story. Not with the setting. Not with a description of the characters, no matter that they are undoubtedly interesting and deserve a lot of space in the story. That’s not what you do. You start with the hero setting forth on the journey that will be the bulk of the story.
Yes, you’ll need all that other stuff eventually. But not at the beginning because the beginning has a couple of jobs to do and there’s only so much that can be accomplished in the brief part of the book that is the beginning.
First the beginning must put forth the premise of the story. The theme. The over-arching idea that is behind the story.
Second the beginning must draw the reader into the story so they don’t put the book down in disgust and not read another page.
That’s a lot. Enough to keep any experienced or beginner writer busy until they are past the beginning and deep into the story itself, which will flow much easier and better because of a good beginning.