Setting your white balance can be done two ways: in camera when you’re shooting or later in post-processing. In this short video tutorial Phil Steele shows you how to do both methods.
Part one is using a gray card to set a custom white balance. Read more about that topic with: Get your White Balance Right in Seconds Using Grey Card.
Part two is how to adjust the white balance using a gray card in Lightroom. Read more here: Adjusting White Balance in Lightroom.
Phil also mentioned where to buy gray cards, you can find several options on Amazon: