this sounds pretty standard, go to the page and have a login that asks for the card-code. this opens a blog-page that can add a message with that card’s code. You could include user-login too, to show the user’s info, though it sounds like a facebook login might be better. You might even add a QR code to the card for a user to simply scan with his phone for quick mobile login.
If you still don’t have any idea, you need to have an actual developer do it, though this sounds like a good simple MVC page project, with something like Ruby on Rails, PHP Codeignitor, or Micrisoft MVC, depending on your preference.