Calligraphy is an easy skill to learn if you're left-handed, although the special tools are often hard to come by. Here's the basic method that I used when learning to write calligraphy with a left hand. I hope it will help other southpaws as well.
First of all, it's important to find out what materials you will need. To start practicing calligraphy, all you'll need is a calligraphy practice pad, a left-oblique cut calligraphy cartridge pen with refills and nibs (not the usual right-handed pen with a straight nib), and a guide book.
Now is the time to gather your materials. The Bienfang Calligraphic Practice Pad is a great practice paper to use, and it's available from MisterArt.Com. Manuscript has a good, inexpensive left-hander pen with nibs and refill cartridges that you can purchase at JohnNealBooks.Com (you can order Manuscript refill cartridges at MisterArt.Com, too). And a great guide for the aspiring left-handed calligrapher is "Left-Handed Calligraphy" By Vance Studley.
Armed now with your materials, read Studley's book from the beginning and follow his directions. He presents learning calligraphy in a logical, helpful progression, from how to position your paper as a left-hander, to the different ways of holding your pen, to making the different strokes that comprise the calligraphy alphabet.
If you can, practice regularly for about half an hour each day; you will ensure your fastest progress with a scheduled practice routine. Most of all, enjoy learning the art of beautiful writing!