How can I make a pop up message appear when the Opportunity Stage changes values?

I'm interested in having a pop up message appear whenever a User closes an Opportunity but am not sure how to do this.  I assume it can be done with Javascript but I'm not familiar with the coding.  I've taken a stab at it in a custom web applet but I know the onChange() line is way off.  I'm not sure how to listen for a change to the variable.

Can someone share an example with me?  Also, let me know if there's an easier way to accomplish this same task.

<script type="text/javascript" id="customJavaScript">

%%%Sales_Stage_-_Translation%%%.onChange(){                    //Check and run function when the Opportunity Stage changes value