Go to https://www.python.org/downloads/ and download the latest Python to your Mac Computer. Go through the download process and move the Python system to your applications folder.
I suggest always using the latest. Some older scripts will be written using older versions of Python, which includes a lot of Python 2, however I always feel that in learning Python 3 straight from the off, you will know the most relevant and up to date version and have the ability to then learn older methods as and when it is needed as time goes on.