How do you start off a descriptive story about the future?

Joyce Carol Oates said it best: "The first sentence can't be written until the final sentence is written."

Worry about finishing your story, not about writing the perfect opening sentence! You can change anything you like once you've finished -- go back and think of a great first sentence then, instead of waiting for some magical "first sentence" to fall out of the sky and hit you in the head.

Now get off the internet and get back to writing that story!