How to Calculate Standard Error?

First, with a set of numbers, calculate the sum of all. Then, square each original number and add to get the sum of the squares. Your average should be sum/n, sum being the sum of the numbers and n if the amount of numbers you have. So, if sumsq equal the number of your squares the standard deviation should be stddev equal sqrt ((sumsq-sum^2/n) / (n-1)) and so forth.