It depends on what you are trying to do with the photograph. You will want a small aperture (large f#) if you want a lot of detail all of the way from the foreground to the background. Conversely, you will want to use a larger aperture (smaller f#) if you want to isolate a subject against a blurred background, say a flower with the faint impression of a snow capped mountain in the the distance.
The same goes for shutter speed. Use a fast shutter speed to freeze action for sharp detail. Use a slower shutter speed to blur moving subjects like water or a field of wind-blown wild flowers.
For ISO, use the lowest for which your camera is calibrated. My camera's lowest is ISO 100. The Nikon D90's is 200. However, the D90's can be set lower than 200. Such setting is shown as Lo1 or such. It is not calibrated to a specific ISO number, but can get you close if you need it. General sticking with the camera's base ISO will give you the best image quality. Increase the ISO if you need more light for either stopping action or depth of field.
Exposure settings ultimately come down to a creative decision. For any given "correct:" exposure, there can be five or six other combinations of aperture and shutter speed that will give you the same quantity of light. However, each combination will be vastly different in creatively with some isolating a single subject with limited depth of field, others having foreground-to-background depth of field, others freezing any action, and others blurring action any action.
No single combination is more correct than the other. The best one is the one that matches your internal vision of the image.
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