As a school-aged person, I would advise you avoid the "white shirt, black tie, backpack" look that has become synonymous with door to door Mormons.
That said, ain't nothing wrong with a tie, though personally, I think it looks MUCH better with a jacket (see: literally all of the pictures in this thread) If anyone gives you grief, just say something like, "You don't have to be upset cuz I look good."
Also, a few more quick points
- Ties with slacks (no jeans, dockers, khakis, etc) and I'd probably keep it in the Black/Grey/Brown range. Maybe beige.
- Jacket matches the slacks. If you ever decide to don the jacket.
- If you want a little more low-key way to still be stylish, rock the button down or polo shirt sans tie. It's a good look, but a little less obviously dressed up than a full on suit and tie
- Tailoring: Not just for WoW anymore. If you've listened to my other points, and opt for the full suit and tie. Get one that fits properly. Having it tailored and drylceaned will be a bit pricey, but it's worth the extra cost to not look like, well, Woodsley's picture a few posts up.