You can download and install Python 3 from https://www.python.org/downloads/release/python-381/. The most recent version is 3.8.1, but any version 3.x will work. IDLE comes with the Python install, so you don’t need to find that separately.

For Mac or Linux, I believe you should already have Python installed. When you start IDLE, the first line says something like “Python 3.8.1” and then a bunch of other stuff before the “>>>”. You can easily use that to check your version. If it is 3.4 or higher you should be fine.

For Windows, you can get it from the link above. I recommend either the “Windows x86-64 executable installer” or the “Windows x86-64 web-based installer”. If your machine is not 64-bit (which is possible, but less likely), you’ll need the installer with only “x86” in it, not “x86-64”.