Well...for some fun, the flash is slow, but...whatever...you can go to www.Behr.com
, and choose the "Color Smart" link under Explore Color. Its pretty interesting to play around with and see different color combinations. Other than that...
Complimentary colors are usually the colors opposite one another on the color wheel. You'll have to figure out what type of primary colors you wish to base this off of first. Red, Green, Blue are your standard additive primary colors your monitor uses. Red, Yellow, Blue (or Magenta, Yellow, Cyan) are your basic print primary colors. If you're going to use the additive color wheel, in photoshop to find a colors compliment, just use Invert.
Other than that...color theory is quite complex, and is better explained with a book, guide, or college course. You'll have to deal with things like warm, cool colors, split compliments, and a bunch of other color combinations that I've forgotten the names for, or exactly what they are.