In the menu, look for the option for white balance. If you do not know where it is, you may need to refer to your manual. Usually the default is auto white balance, which works well in a number of situations. Then you just choose the white balance that you want.
For example, if you are taking a picture under bright daylight, you may choose daylight as your white balance. If you are taking a picture under sunset, you may choose to use the sunset white balance. Suppose you wish to emphasize on the orange tinge of the sunset, however, you can choose to "fool" the system and use daylight for sunset photos.
If you wish to use custom white balance, use a white card or any white surface that you wish to appear white in your picture. Take a picture of the white surface. Depending on the camera you are using, you may need to fill the frame with the white surface. Choose the custom white balance option, and choose the picture that you have just taken for the white balance.
I hope that helps. And experts, please correct me if I explained anything wrongly.