How do I add a pop up message when my form opens?

I'm new to this so I apologize now. I created a form in LiveCycle 8...no problem there. What I need to know is how do I add a pop up message when the form opens. what is the JavaScript needed and where do I put it. I was told that because the form was created in LC I cannot create an app.alert in Acrobat Pro; I have to do it in LC. Thanks!