how can I make a welcome popup window upon visiting the website
You are looking for Splashify module, it also have the option to popup on any page.


  Display a splash page anywhere on your site by redirecting the user,
  showing a lightbox, or displaying a popup. This module is designed to
  be search engine friendly, mobile device friendly and offers multiple
  configuration options.


or 

You can try TinyBox (Simple Splash). see the demo


  The main purpose of this module is to provide Splash Screen/Window as
  simple as possible.


or 

take a look at Splash Offer module


  Allows the management of one or more "splash offers", which are modal
  popups with an accept or reject form. If a user accepts the offer then
  a new page will open with a provided accept url. If they reject the
  offer, the modal will close.