Using Python on a Macintosh explains how Python 2.7 is installed by default on Macs. You should start setting up your environment by installing Python 3 and then follow the instructions in the post to get a development environment configured.
Python Development Environment on macOS High Sierra takes the defualt macOS environment and gives you the step to configure everything for Python development.
How to Set Up Your Python Environment is a data science-flavored configuration guide.
How to Use the Command Line for Apple macOS and Linux explains the Terminal. If this guide is useful to you then you should also check out the shells and Bash pages.