How do you frame your image? What do you put in? What do you leave out? What do you focus on?
Do you fill the frame? Should the light be soft or hard?
You'll see some rules, but you don't have to follow them all the time.
For example, the "rule of thirds" says that if your frame was a tic-tac-toe board, then subjects should be on one of the lines of the board. Your scene is divided up into thirds. But if everybody followed this rule all the time, then I think this rule wouldn't exist, and we'd miss lots of great photos!
How to Compose Photographs! #1 (10 tips, easier)
How to Compose Photographs! #2 (20 tips, more detailed)
Ten Rules of Composition (TechRadar)
Clever Framing (Eric Kim)