The first thing you need to do is practice in front of the mirror. You can even have a friend take some preliminary photos with a disposable camera. Learn to play up your features. Try looking left, right, head on, with your chin slightly raised. One common myth is that models are taught to hold their heads down- that's a big no-no. Holding your head down can give you a double chin or a bigger nose. Likewise, you don't want to hold it too high or too far back. Try tilting it in different directions. Play around with your facials- smiling, serious, coy, sexy. You may look perfect if you smile with your head tilted to the left, but look horrid if you give a sexy look in the same manner. The camera is strange like that. You have to learn your own personal attributes and how to work them.
Step2
Hair and makeup should both be simple. The shot should look just like you. Avoid glamour shots. Casting directors tend to throw them directly into the bin. They want to see what they have to work with, not what a lot of makeup can give them. Wear makeup, but don't go overboard. This is not a fashion shot, so forgo the big jewlery and accessories- they take away from YOU.
Step3
Wardrobe and background should also be kept simple. Shirts with loud patterns are distracting. If the the viewer sees a magenta shirt with stars before seeing your face, the headshot is not a success. A simple background will ensure the viewer's attention is focused on YOU.