Hello, I an a newbie. Does anyone know how to add a popup window so that when someone visits my site a window will appear to tell visitors of new products or information?

People don't like popups.
The newest versions of the most common browsers (Firefox, Safari, IE) are blocking popups by default due to spamming and security.
Hence popups are not a good idea.

You better publish the news somewhere on your website (first page). You could use an iFrame like I do for the measse at the top of all my pages and the menu displaying on the right of all my pages.

Regards,
Cédric