Start by realizing that you’re going to have to condense your story. The average book is typically between 60k – 80k words. You have to take it from what it is and get it down to around 100 pages. So you have to discern what is the main throughline of your story.
Film is a visual medium, so use that to help make the cuts. You can’t show the internal thoughts and feelings of a character the same way you can in a book. There’s no narrative summary here. The only thing that goes up on the screen is dialogue and action. Nothing else can cross over.
Also, and I can’t stress this enough – dump a lot of your book’s dialogue. In books, you can have a lot of dialogue that is fluff at its core. In a movie, every word should be utterly necessary. Take it down to its essentials and scuttle the rest.
Now, none of this means you don’t write some prose. Keep in mind, you have to get past a script reader. The script needs to be an enjoyable read, so don’t just write boring action lines and dry descriptions. Have some fun with it.