'Publish' button in Google Spreadsheets to make HTTP Request to Heroku

I'm a newbie in web engineering. I'm trying to build a project that whenever my google spreadsheet file updates, my github repository will update the file as well. It works like this:

1) Google spreadsheet creates/updates the file

2) I click "Publish on the web" button

3) Button makes HTTP request to my Heroku app

4) Heroku commits a CSV file containing google spreadsheet contents

5) Github repo is updated

My question is, how do I do steps 2, 3, and 4? How do I make the clicking of the "Publish on the web" button in Google spreadsheet make an HTTP request to my heroku, to get contents and make it into CSV variables? Thanks!