Determine if you want the lowest decile or the highest decile, that is, are you looking for the highest 10 percent or the lowest 10 percent?
Sort your data from highest to lowest.
Multiply the total number of data points by 0.10 (10 percent) for the number of data points in the top decile. For example, if you have 10 data points then 10 x 0.10 equals 1. This means that your highest (or lowest) number represents the numbers in your decile.