This is a collection of introductory videos made for T3D.
The Scene Construction series guides you through all of the World Editor, and teaches you how to build a level from scratch.
Rene Damm has made an introduction to Zones and Portals:
Jared Swets made a bunch of videos, covering:
learnTorque made a pair of very basic introductory videos, which are a couple of jack-of-all-trades tutorials:
PRDV made a bunch of videos, covering:
BigDaz made a video on how to create grass.