Create a new module in the template using the VBA editor. Create a macro
named AutoOpen if you want the popup to happen every time a document based
on the template is opened, or AutoNew if you want the popup to happen when a
new document based on the template is created.

What do you want this popup to contain? is a simple messagebox enough?

I assume that by "when a user opens a particular template" you mean that
"when a user creates a new document by selecting the template in the
File>New dialog.

If that is the case and it is just a message that you want to display,
create the following macro IN the template:

Sub Autonew()

Msgbox "This is my message."

End Sub

If however you need the user to be able to input information, you are better
off to use a userform.

See the article "How to create a Userform" at:

http://word.mvps.org/FAQs/Userforms/CreateAUserForm.htm