There are several algorithms implemented in MATLAB to perform detection of objects. The following link lists the different ways in which it can be accomplished in MATLAB:
In particular, since you mentioned that you are interested in training an object detector, you could refer to the "Cascade Object Detector" using Viola-Jones algorithm. The following link describes how to train a detector to detect objects of our requirement:
Furthermore, after the detector is trained, refer to the link below that describes how "vision.CascadeObjectDetector" system object can be used to perform detection on a test image:
The following link could also serve as a good starting point for Object recognition:
An important point to note is that these functions are available only in Computer Vision System Toolbox.
