I recently launched InstallPython3.com, a site solely dedicated to providing accurate, beginner-friendly guides to installing Python on Mac, Windows, Linux, and Chromebooks.
If you go to Python.org, the advice is to download the latest version directly onto your computer, then configure PATH, and so on. I don’t find this to be very friendly to newcomers. On a Mac, using a dedicated package manager like Homebrew is far friendlier in my opinion, both for installs and updates. So that’s what I recommend here.
On Windows, as of May 2019 it’s possible to install Python 3 directly from the Microsoft Store. This has a number of advantages and is, again, easier for installs, updates, and not messing with other packages on a machine.
Then we come to Chromebooks, which are used in the majority of secondary schools in the United States. Many students are exposed to programming for the first time via Python on these machines. While there are a number of quality web-hosted platforms such as repl.it and Glitch that work well on shared machines, if you do have your own Chromebook, which are relatively inexpensive, it is now possible to run them in Linux mode, install Python directly, and have a moderately effective development machine. I see this as a great equalizer in opening access to programming so I hope to add/improve this section over time.