The following guides include all Diptera families that have been described from any part of the world. Start with "Diptera - Introduction and outline," for an overview of the differences between the five main subgroups of true flies. 

Diptera - Introduction and outline

Nematocera - families

Orthorrhapha - families

Aschiza - families

Acalyptratae - families

Calyptratae - families

Key to suborders of Diptera

Larvae

Nematocera - families - LARVAE

A good introduction to common fly families:

http://phorid.net/zadbi/education/how-to-identify-flies/