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I'll explain how to do the following problem:
Find the volume of the solid generated by rotating the region bounded by y=f(x), for x belonging to [a, b], around the x axis.
(Your problems are the same, you just specify f(x), move the axis of rotation, etc.)

A small region of [x, x+dx] will generate a disc with the thickness dx and the radius of R=f(x), its voulume dV=pi*R^2*dx. The whole volume of the body will be the summ of all such small volumes (I omit the rigorous approximation and taking the limit approach), or in the limit of dx going to 0 an integral:
V= Summ of dV = Intgeral (from a to b) { pi*f(x)*f(x)*dx }