Raspbian is based off of Debian Linux and is the official Operating System made specifically for the RaspberryPi. So go a head and head over to https://www.raspberrypi.org/downloads/ and download Raspbian! You have two options Raspbian Stretch with Desktop or Raspbian Stretch Lite. The later does not have a desktop environment and is terminal access only! Refer to the offical RaspberryPi Image installation documentation as required to compete your setup!
Don’t forget to open up terminal app and run “sudo apt-get update && upgrade” after your setup is complete to ensure everything is up to date!