Put simply, macro photography is close-up photography. The G12 features a Macro mode that you can enable in most shooting modes. (The Auto mode, for example, switches to Macro if it senses objects are close to the lens. The Low Light, Quick Shot, and some of the Scene modes do not support Macro.)
You’ll likely want to work with a tripod, because handholding can make focusing difficult. Also, I recommend shooting in Av mode so you can achieve differing levels of depth of field. If you are shooting outside, try shading the subject from direct sunlight by using some sort of diffusion material, such as a white sheet. By diffusing the light, you will see much greater detail because you will have a lower contrast ratio (softer shadows), and detail is often what macro photography is all about (Figures 10.8 and 10.9).
To enable Macro mode, press the Macro button in most shooting modes. A large flower icon appears. Get close to your subject (be careful you don’t nudge it with the lens!), focus, and shoot.