This is presently a work in progress. Until it's written, you should start here:

http://www.freepbx.org/support/documentation/installation/first-steps-af...

Other Documentation Sources:

1. Elastix Without Tears
2. PBX In A Flash Without Tears
3. PBX In A Flash For Dummies
4. PBX In A Flash Documentation
5. NerdVittles.com
6. PBX Setup Guide
7. Voip-info wiki - Contains a lot of documentation about VoIP in general, as well as many Asterisk-specific things.
8. Asterisk Primer
9. Asterisk: The Complete Guide, 3d. Edition

Other Help Sources:

1. FreePBX Discussion Forums
2. DSL Reports VOIP Tech Chat Forum
3. PBX In A Flash Forums
4. The Michigan Telephone Blog