That's a much bigger question than can be answered here. You need to develop an understanding of anatomy, sculpture and portraiture before you even begin - draw your face and your features from multiple angles - then you need to understand the process of modeling and developing good topology. There are literally hundreds of tutorials on forums like this one, on Digital Tutors, on Lynda.com - each has their own methodology, tricks and tips.
Probably the best method (in my opinion) would be to do a high-res sculpt in Mudbox or ZBrush, then retopologize the model. Another way that can get you some great results would be to use Autodesk 123D Catch to get a high-density mesh of your face, then retopologize it.
Good luck.
