I have been working on my own Atc's. I am a "newbie" at this, and the whole process is a little daunting, but also very addicting. There is lots of help along the way, however! First, there are many wonderful images on flickr. All you have to do is search atc and lots of wonderful images will pop up. Googling, "how to make atcs" will also bring up multiple guides and resources for making these little works of art. You can use just about any media you want to make an atc. I tend to want to use fabric or some sort of fiber in my cards, probably because that is what I am used to working with!
Here is a series I just finished. The challenge was to use a brown bag on most of the card. I had a lot of fun sewing the newsprint to the fabric.