Let item A be the good we have developed a demand schedule for and item B be the related good. If the price of item B goes up and, as a result, the demand for item A increases; then items A and B are substitutes.

If the price of item B goes up and, as a result, the demand for item A decreases; then items A and B are complements.

Name some substitutes.

Mountain Dew and BeerApples and Oranges

Name some complements.

Beer and Beer Nuts
If the price of beer increases you will consume less Beer Nuts
Gasoline and automobiles