what is variance? and how do i get it

This page discusses variance and its calculation:

http://www.mathsisfun.com/data/standard-deviation.html

Variance in the field of probability is measuring how far a set of numbers is spread out. A variance of zero shows that all the values are identical. Variance will always be a postive number. Always when the variance is small, it indicates that the data tend to be very close to the expected value and therefore close to each other, while a high variance indicates that the data are very spread out around the expected number and from each other.