On most versions of MacOS before Catalina, a distribution of Python is already included. Unfortunately, it’s almost certainly an old version, Python 2.7. Luckily, there are two ways to install Python 3 on a Mac with ease.
First and foremost, I recommend looking into Homebrew. It allows you to install almost anything easily. The added benefit is that it’s also easy to upgrade to newer versions later on.
Once you are up and running with homebrew, installing Python on MacOS is as easy as:
$ brew install python
Alternatively, you can download an installer from the Python download website. The downside to this approach is that you won’t get automatic updates. Other than that, this option is fine and it might be the easiest choice if you didn’t already have Homebrew installed.