The topic is very involved, so I am only going to cover it very basically. The object of achieving an average correct exposure is for the film to register human skin-tone as accurately as possible. Sometimes this involves a compromise exposure, or sometimes you choose to expose just for one subject. The key instrument in exposure selection is the light meter. To take an incident reading, place the lightmeter next to the subject, and point the dome towards the camera.
Reflective meters read the light that is reflected from the subject.
Many light meters serve as both reflective and incident meters. When using such a model as a reflective meter, remember to remove the white incident dome. Sometimes these meters require you to read them differently when used in different modes, so be sure to read the instructions. Many cameras have a built in reflective meter, in which case you use them the same way as a normal reflective meter. Remember, light meters read the amount of light only.