Having been the model for years before I ever picked up a camera, here's what I wish every photographer had told me before a session. I'm writing it now from the other side.
First: you don't need to know what to do with your hands. The photographer's job is to notice a gesture that's already yours and make space for it to happen again. If they're asking you to pose, they haven't found it yet — that's on them, not you.
Second: the camera shutter is loud. You will hear it and flinch, and the first thirty frames will look stiff. That's fine. Some of the best portraits I've taken are frame 120-ish, when the sitter has just forgotten I'm there.
Third: wear what you actually wear. Stylists sometimes borrow clothes that look great but sit wrong on you. You are not the clothes. You're the person inside them.