How can I set up an existing site from Pantheon on my computer? I am a real newbie to all this so I appreciate all the help.

I am helping a company develop a Drupal website which already exists and is hosted on Pantheon. I have login access to the drupal site. How do I get the current codebase for the existing drupal site?

From what I understand, Pantheon automatically commits codechanges to the development section. However pantheon seems to use git so I downloaded git and then created an ssh key and pasted it into my account on pantheon. So I think I have created the connection between my laptop and pantheon. 

So really all I think I'm missing is how to extract the codebase. 

Thank you and let me know if I'm missing a whole bunch of other steps.

By the way do I have to use git? Or can I use github?
Uploading your key is the first step towards cloning your repository because this gives you access. The next step is to grab the git repository address from your site's dashboard. This is located in the "Connection Info" dropdown menu on the upper righthand side, or in the "Connection Mode" bar above your commit log. The info button provides the git commands you'll need to get started. Please note that you'll need to be in the Git mode in order to push your local commits to the Pantheon repository.