Hey badams.

To calculate the standard deviation (or sample error), you need to have either a population distribution or a sample of observations from that distribution.

Calculate the sample standard error of a sample is basically 1/(n-1) * sum(x_i - [x])^2 where [x] is the mean of the sample and x_i is the ith observation summed over all observations (basically sum of squares divided by n - 1).

This can be used to estimate the standard deviation of a sample, but you still need to decide whether you have an assumed population or whether you don't and these are two very distinct things using very different kinds of statistics (but the same core principles do apply).