First, check if git is installed on your system. Pay attention to the version you are using. If you would like to update to a newer version the easiest way is to use Homebrew. You don't have to update to a new version if you don't want to.
git --version
brew info git
brew install git
Windows users, sorry, this will be a little more difficult. I find that the easiest way to install git is to download git bash from the git website. Once this is installed we can use git bash as a unix command line emulator. Which means all the commands I run on my mac should be the same for you.
I trust you know what you are doing! If git isn't on your system you'll use your distros package manager to install git.