You'll need to remember whether the user has already visited the site, and only show the message if they haven't.
If your server-side code needs to know whether it's the user's first visit, you'll need to use a
cookie[
^].
If you only need to know from Javascript, then use local storage:
Window.localStorage - Web APIs | MDN[
^]
Local Storage And How To Use It On Websites – Smashing Magazine[
^]
However, if you're trying to open a new browser window (
window.open[
^]) automatically when the page loads, that will not work. Your popup will be blocked by the popup blocker built in to every major browser. Instead, you'll need to show your message within the page, using something like
jQuery UI's Dialog[
^] or
Bootstrap's Modal[
^].