The Raspberry Pi Boards have a row with GPIO Pins.
On the picture here on the right you can see what it looks like.
The selected pins at the top left use if you want the fan to always be on. (as long as the Raspberry Pi is on)
The left pin is the 5V and the right is the GND
What happens if you don’t connect a fan?
If you haven’t connected cooling on your Raspberry Pi, and you’re working normally, you’ll probably soon get one of the two notifications that are in the picture right here.
If you ignore these warnings, it is possible that the Pi will shut itself down.
Temperature warning 80 – 85 C°
Temperature warning +85 C°