In the example of a green apple below, the light is coming from the right-hand side, at about two o'clock. So the highlight will be on the top, right of the apple, and the shadow at the bottom, left.

Adding a highlight: Highlights occur where bright light hits an object and are usually quite small. Don't use pure white to create a highlight, but rather a very pale shade of the colour of the object. Pure white will generally appear too harsh or artificial. So in the case of this green apple, use a very pale shade of green.

Adding a shadow: Shadows occur on the opposite side of an object to the highlight. Don't use pure black to create a shadow, as this will tend to look stark and harsh. Rather use a dark shade of the main colour of the object, in this case a darker green. Using complementary colours for shadows: You could also use a dark shade of the complementary colour of the object. The reason for doing this is that complementary colours are said to make each other appear brighter. For a green apple, a shadow in a complementary colour would be a deep red; for a red apple it would be a deep green. The easiest way to get a dark complementary colour is to add mix some black in with it.