@Herb_91: Great topic and good on you for taking a newcomer on their first backpacking trip! I am sure there is a lot of good advice out there but I would suggest a couple of things:
1) Know the person you are taking well enough to keep the trip well within their abilities. At the end of the day we want them to have fun, be safe, and want to go again!
2) If possible pick a trail with a cool payoff: stunning vista, alpine lake, waterfall, etc. One of my first backpacking trips was through Hole-in-the-wall in Olympic National Park and I still talk about it to this day!
3) If the person isn't really an 'outdoors person' keep that in mind as you can suggest creature comforts (pillows, lightweight backpacking chairs, a certian type of food, etc) as well as avoiding, as much as you can, any real fears the person may have (bears, bugs, etc.).
Thanks for helping people get outside and enjoy nature!