This guide uses Raspbian Stretch Lite as the starting point. Download the latest version from here:
You will get a .zip file. Unzip that and you should get a .img file. Then follow these excellent instructions to burn the OS image to an SD card:
You live in a world where an entire operating system can fit on a wafer thin piece of plastic smaller than your finger tip. And you can run this on a $5 (or $10) computer that is small enough to give away on the cover of a magazine. You should be amazed, excited, and happy about this.