Variance is just the square of the standard deviation, and can be calculated in a variety of ways. The usual way is:
Variance = E(xi-xbar)^2/(N-1)
This means that variance is the sum of the squares of the differences between the values and their mean, divided by the degrees of freedom (or number of values minus 1).
To put this in other terms, you need to find the mean, and then subtract all of your values by the mean. Once you do this, you must square these differences. Add all of these new squared values together, and divide it by one less than the number of measurements you have.