The easiest way to get Python and the ability to install a wide universe of related packages is through Anaconda. Easy graphical installers for Windows and macOS, are available directly from Anaconda.
You should usually install Anaconda for the local user, which does not require administrator permissions. You can also install Anaconda system wide, which does require administrator permissions – but even if you have those permissions, you may find that installing only for one user prevents problems arising over multiple users editing common packages.
If you already have Python installed, either by itself or as a companion to any one of a variety of common transportation planning tools (e.g., ArcGIS), you can still install and use Anaconda. You do not need to uninstall, move, or change any existing Python installation. Just use the standard Anaconda installer and let the installer add the conda installation of Python to your PATH environment variable. There is no need to set the PYTHONPATH environment variable.
Once Anaconda is installed, it can be accessed from the Anaconda Prompt (on Windows) or the Terminal (linux and macOS).