I did a tutorial on modes here:
http://www.guitartricks.com/tutorial.php?input=370

The church modes can be useful in understanding and tracing the historical development of tonal theory. But, it isn't absolutely necessary. Personally I think the best place to start is JJ Fux's book on counterpoint.

I did tutorials on counterpoint, harmonic conception and voice leading here:
http://www.guitartricks.com/tutorial.php?input=415
http://www.guitartricks.com/tutorial.php?input=147
http://www.guitartricks.com/tutorial.php?input=167