A good way to start is by reading about Principal Component Analysis (PCA also known as Eigenfaces), Fisher's Linear Discriminant Analysis (LDA), and Support Vector Machines (SVMs). These are classification methods that are extremely useful for face classification.
OpenCV already includes excellent implementations of PCA and SVM. An excellent resource for face recognition and classification code for OpenCV in C++ is this website.
One website that offers resources and links to papers for most relevant methods for face classification is this one.
A well explained example of PCA Eigenfaces and LDA with sample code in Matlab that was extremely useful for my first face classification program is here.
