Hi Devon,
The best way to learn is from direct
experience with birds. Just go and sit on your porch while you drink you morning coffe and observe. One day you might notice a bird that your recognize do something out of the ordinary. Then you might realize the next time that you see that behavior that it is a reaction to something, a neighborhood cat for example. Now you can tell when a cat is going to come into your
yard. That's bird language.
When you're watching songbirds you're looking for 2 categories of vocalizations; baseline, as in everything is ok, and alarm behaviors. The baseline vocalizations are; Song, Companion Calls, Juvenile Begging, and Territorial Aggression. The 5th voice is alarm. You can tell what type of predator is causing the alarm by the way the bird alarms.
I recently finished a DVD called Bird Language with Jon Young. It's 2 disks with a laminated reference card that teaches the basics of bird language and how to setup a group. It sells for about $40.
I'm also soon going to be posting a few new videos on the youtube channel describing the vocalizations of our '5 best bird language birds'. Look out for that in the next copule weeks.
Also, my friend's website has a good library of recordings of birds with descriptions about what they are "saying" here:
Bird Language Library
Jay is also right. There are lots of good instructors. Where are you? I might be able to hook you up with a community of people practicing bird language.
Cheers,
Calen