The most basic building block of any thrown piece is the cylinder. Everything can be made from a simply cylindrical shape, and once you master making these well, you could make almost anything.
Once you have the ability to craft cylinders of different sizes the trick is turning them into a more interesting shape. Shaping in general has to do with two things, bellying and collaring. Bellying a cylinder is like stretching it out, you run your inside hand up the walls and push outward to stretch the clay and widen the cylinder. Likewise, collaring a cylinder makes it narrower by squeezing the clay evenly around its circumference. Things can sound much easier than they are by just talking about them, but with practice a potter can create just about any shape they want with only a few movements.