Do you have a shiny new camera in your hands? Digital Photography School has a list of 5 experiments to do to get to know your new
Most interesting to me was this one.
White Balance can be trickiest when there is more than one light source and it’s helpful to know which way to adjust the colors. A lower temperature (incandescent and fluorescent) give more of a yellow/green cast while cloudy and shady gives your more of a blue tone. If you want to eliminate that color cast in your images, move the White Balance toward those settings.
White balance has always been one of those settings that I just leave on auto as it’s usually good enough.