How to get notified when a specific paper is cited
If you asking this for your paper(s), then this is procedure:
Go to your Google Scholar profile, and click on some paper/article title. Now you will have one new window open, in which you have to click on the "Cited by" link at the bottom left of that window. Finally, you want to click on the envelope icon in the left sidebar to create alert... The similar procedure will work on papers from other authors!