A typical headshot portrait is cropped to head and shoulders, showing the whole of the face. In the studio, this will be on a plain background, white, black, and gray being popular choices.
Most people, even actors, can feel awkward and self-conscious when posing for a photographer. It's good to practice posing in front of the mirror. Bear in mind, most people's faces are unsymmetrical which leads to a 'good' side and a 'bad' side.
Our photographers prefer shoulders at 3/4 profile to the camera with you head turned towards the camera so that both eyes can clearly be seen. Leaning slightly forward from the waist will cause the neck to elongate creating a strong and confident image.
Squinting ever so slightly with the lower eyelid may improve the look of the shot. It's important not to overdo the squinting unless you are trying to get a part in a western.