The standard error of the mean (SEM) (assuming statistical independence of the values in the sample) is estimated by taking the standard deviation of the population sample, divided by the square root of the sample size: $$se = \frac{{s}}{{\sqrt{n}}}$$
References
Kenney, J. F. and Keeping, E. S. (1951) Standard Error of the Mean. In Mathematics of Statistics, Pt. 2, 2nd ed. Princeton, NJ: Van Nostrand, pp. 110 and 132--133.