Instead of burying the instructions for your application deep in a FAQ somewhere, or trying to make them sit through a long tutorial during the signup process ("I just want to use the app!!1"), do what YouTube does:

Wait for them to take an action, then encourage them down the right path with more details. For infrequent and big/meaningful actions like becoming a contributor to YouTube (as opposed to most users who are viewers), an email is a great approach. For more common actions, in-app tips are better. The user has taken an action, and that's a great time to teach them something new.

This post on training users has been brought to you by Cesar Millan (not true) and National Geographic (also false).