Many developer tools (including Git) are installed as part of the XCode package. If you haven't already installed Xcode for other development work, you can download it for free from the Mac App Store. Once installed, you should also set up the command-line tools—to do this go to the Xcode menu and select Preferences, then Downloads and Install Command Line Tools.

If you're unsure about whether or not you've installed these already, then open the Terminal application and execute xcode-select—if installed, that will execute normally, and if not, you'll be prompted to run the installation:

In addition to these tools, you'll need to install Go. You can get the download package from https://golang.org/dl/—tap the featured download link for Apple macOS and run the installer package that downloads. You may need to close any open Terminal windows to update your environment variables.