Differentiate the moment-generating function in Exercise 3.147 to find E(Y) and E(Y^2). Then find V(Y).

Exercise 3.147

If Y has a geometric distribution with probability of success p, show that the moment generating function for Y is

m(t) = pe^t/(1-qe^t) , where q= 1-p

Im confused... Differentiate with respect to what? After I take the derivative, how do I find E(Y), E(Y^2) and V(Y)?

Thanks