A pie chart is used to represent quantities in proportion to other quantities. i.e. To show percentages of a whole.
To make a pie chart, simply take a percentage of something (eg. coca-cola's share of the cola market: 51%), turn it into a decimal (0.51) and multiply it by 360 (183.6).This result is the number of degrees to partition off to visually represent the amount, i.e to make the "slice." Repeat this process for other values, using a protractor if necessary.