It sounds like you are asking about the standard error - the standard deviation of a sampling distribution. (In this case, you are interested in the standard deviation of sampling distribution of the mean.)
If you know the standard deviation of the population (sd) and you know the number of observations in the sample (n), you can compute the standard error (se) as follows:
se = sd/sqrt(n) where sqrt refers to the square root
This is explained in more detail (and maybe a little clearer) at http://stattrek.com/Lesson3/SamplingDistributions.aspx .