Your photographer, if he or she is highly experienced, should be able to give you excellent instruction and direction in order to get great shots. Still, it is very helpful if you have at least a basic understanding of what your body looks like in various positions, and to understand how your face feels when you are expressing different emotions.
All first-time models, and anyone relatively new to modeling, should practice posing and making various facial expressions in front of a mirror. You might feel silly practicing these things in front of your mirror, but it goes a long way to preparing you for what it will feel like to be in front of a camera making those poses and expressions. This works great for making your actual photo shoot easier and more comfortable.
Be sure to practice a variety of different looks, to make yourself prepared for when you are asked for a “sexy”, “intense”, “pouty” or “serious” expression. A photographer worth his or her chops will do whatever it takes to get the look and expression they want, but the more prepared you are, the easier it will be for both of you.
Finally, if you have the tendency that some people have of showing a lot of your gumline when you smile, practice smiling without showing so much of your gums. Sure, you’re going to get plenty of photos with your beautiful and natural smile, but professional photography tends towards limited exposure of the gumline.