Can you please explain what is differentiation and how to do it?

Differentiating a fuction ot a variable means calculating the change in that function or variable with the respect to the change in the variable with respect to which is being divided.

OR

Derivative is a measure of how a function changes as its input changes. Informally, the derivative is the ratio of the infinitesimal change of the output over the infinitesimal change of the input producing that change of output. Geometrically, for a real valued function of a single real variable, the derivative at a point equals the slope of the tangent line to the graph of the function at that point. The process of finding a derivative is called differentiation.

Please go through
http://www.askiitians.com/iit-jee-differential-calculus/differentiation/