Try playing around with the HSB colour selection model. In HSB (Hue, Saturation, Brightness), Hues (H) are laid out in a circular fashion, with their H values varying between 0 degrees (red) and 360 degrees (red again). If you want a colour's complementary hue, sample its H value and add 180 degrees to it (or subtract 180, whichever gives a valuw between 0 and 360) to get the complementary. Thus, red (H=0) has a complementary in cyan (H=180).
If you want a colour's full complementary, you should also invert the S and B values, which are expressed in precentages. For example, a dark desaturated red (read: brown) has a complementary in a light, saturated cyan.
HSB is available in the colour pickers of most (if not all) Adobe software.
This is only one method. There are multiple other colour models, with their own sets of complementaries. The classic complementary pair red-green, for example, are not complementary in HSB.
If it's creating colour schemes that you're after, you should try more than just complementary colours: there are multiple ways of looking for hues and colours that go well with a base colour. Try playing around with Illustrator's Color Guide (
Color Guide or Shift+F3), where you can choose a base colour and multiple 'harmony rules' to create a scheme. You can try Adobe Kuler for an online version with even more customisation options.