Usually faces are of three shapes -- round, long and square.
If your face is round, add length to your face by having fullness on the top of head. It can also be achieved by either having no parting or one which is placed to one side. Avoid styles where hair is very long and straight or flat at the top of your head.
For a long face, the forehead, cheek bones and jaw are often of equal width and this can be hard, so a soft hair style is usually more flattering. Avoid center parting and dead straight shoulder length hair but aim at a short one.
Square face has an angular shape and any hairstyle should be gentle to soften the outline. Shoulder length hair, flicking at the ends or straight, is flattering and so is a short style flicking away from the face at ear-level. Hair swept to one side or with a diagonal fringe is attractive.