Important I'd recommend you use at least a 16GB sd card. I tried it with an 8GB card and it baaaaarely fits. I had to uninstall a lot of packages and regularly clean up.
Go to https://www.raspberrypi.org/downloads/raspbian/ and download the Raspbian Stretch image (either one).
Use Etcher (from https://etcher.io/) to "burn" the image onto the SD card.