Do you have a color wheel? When I first started stamping and scrapbooking (over 11 years ago), I relied on a color wheel for almost everything. It showed me color combinations that I never would have thought of.
I also used to use the "Gallon, Quart, Pint" method. Your main color is your "Gallon," and it is the one you will use the most. Next is your "Quart," or a secondary color that you use a little bit less than your main color. Last is your "Pint," and you use that just a little bit.
I hope these help you as much as they have helped me over the years. Good luck!