How best to use your camera's spot meter function really depends on the scene, the lighting, and what you want to accomplish or emphasize in the image. For example, if you are photographing a backlit person of average caucasian complexion, you could spot meter their face, and then open up 1 stop, or perhaps as much as 2 stops. In contrast, metering a moderate shadow area in a landscape, you might close down a stop or two. Metering green grass in a sunlit scene, you're probably safe in using that exposure directly.

Like any other typical meter, the spot meter in your camera will give you the exposure needed to render that area as a medium gray (or, the color equivalent). Knowing that, you can make adjustments as needed to get the tonality of the subject correct. Experimentation and note taking will help you nail future exposures.

The camera's manual should tell you what area of the viewfinder the spot meter covers. You can also test that by metering a white area surrounded by dark, noting where the meter reading starts to change.