The official documentation is at: http://docs.alfresco.com
This page serves as a guide to the Alfresco Wiki and external resources related to Content Modeling.
For an overview article describing content modeling, including an example tutorial, you should start with Data Dictionary Guide. After that, continue on to Displaying Custom Metadata to learn how to expose content model properties in the Alfresco Web Client. After that, you can move on to other articles; here are the articles tagged as being related to
Additionally, Jeff Potts wrote a great series of tutorials explaining various aspects of content modeling. These are recommended as a great supplementary resource. They can be found at Jeff's Site - http://ecmarchitect.com.
Additionally, Alfresco and its Training Partnes (depending on geography) offer an instructor-led Content Modeling Course with many hands-on labs. If you are interested in classroom instruction, please contact Alfresco at firstname.lastname@example.org.