Great question! I think this is something that all writers struggle with. My approach when I don't know where to start? I start in the middle. Sometimes you have core parts of your story or argument fully developed in your mind, even though you haven't figured out how to lead into it. In cases like this, I find it helpful to start with what's ready to be put to paper. Doing so often presents beginnings I hadn't thought of previously. YMMV, but I hope this helps!