How do I copyright the photos that I have taken?

See U.S. Copyright Office

Good online resource:
Copyright - Resource Center

HA HA I know this is cheesy, but I just get my name printed on the back of my prints. Most printers will not copy them then out of respect for the profession. Also, when posting my digital pics online, I just make a smaller copy and do a watermark of my name across the print. That way, the copied work is blurry due to lower resolution and unprintable and also has my name across the entire photo! Works like a Gem!