Maybe you've heard a teacher say this analogy, maybe not, but a paragraph is kind of like a hamburger.  Look at the picture on the left and you'll see what I mean.

This main idea is expressed through three sections of a paragraph:

1. Beginning - Introduce your idea.
2. Middle - Explain your idea.
3. End - Make your point again (this is also where you will transition to next paragraph if your writing goes beyond one paragraph).