The
NWTF's website (already mentioned) is a great help. Being that I've just been calling for a few months, I was recently asking the same question. If you are interested in using a diaphram call,
Turkey Hunting Secrets may be of some help. It has quite a bit of information on getting started and how to make some noise, it also has various audio files to download. You'll probly find that everyone will have a slighty different method on how to make the same sound using a diaphram call.
For box calling, I think simply read up on it, listen to some turkey's and then try and master it. But like I said, I'm no master hunter, but I have been right there where you're at... it can be confusing.
Hope I helped, sorry if I didn't.
-Jay