Basic Algorithm for Image Classification
Have a look at this report by Marques (PDF). In particular, have a read through section 1.4. It describes several different aspects to the face detection problem. You might want to decide what scenario you're after. Fully unconstrained (uncontrolled environment) face detection is hard to do accurately.

As a first approach, try just doing color-based segmentation.  See this paper by Singh, Chauhan, Vatsa and Singh for some details about how to use color to segment your images for face detection.