How does one go about copyrighting their own photographs?

I was just looking at some lovely photographs on a woman’s Pinterest page. They were her own work, and next to her name was the letter “c” in parentheses. I assume that means that those photographs are copyrighted.

How does one go about getting their photos copyrighted? Also, how do you put a digital watermark on a photo that you want to post online?

Since I take a lot of photographs, I thought that one or the other, or both of these things might be useful for both Pinterest and Facebook.