Where can I find information about WhatsApp, Skype, Angry Birds, Twitter, NimBuzz, Viber, Fring, or some other non-Microsoft application (3rd party application)?

For information and support, check the vendor’s website.

If you have a Lumia phone, check the Microsoft Store for download. If you have some other phone, check the Opera Mobile Store. Easiest way to download the application is to use Store app (or Marketplace) on your phone.