How do I get an email alert when my paper is cited?
You can set up an alert on Google scholar that will do exactly what you want. From Google's help page:


  How do I get notified when a particular paper is cited?
  
  Search for the title of your paper, e.g., "Anti de Sitter space and
  holography"; click on the "Cited by" link at the bottom of the search
  result; and then click on the envelope icon in the left sidebar of the
  search results page.


An even more effective way can be to set up a google scholar citation profile and select "Follow new citations" from the appropriate checkbox.