Assuming that you are in the US, in theory you own the copyright to everything you make automagically, but to actually enforce your copyright through a lawsuit you must first register it. So if you plan to sue people for copying your website you do have to register it, although you do own the copyright to it even if you don't register.
Read: http://www.copyright.gov/help/faq/faq-general.html Registration is $35: http://www.copyright.gov/eco/
You need to make sure people know that you are claiming copyright. You do this with "(c) copyright your name 2010 - 2011)" or something like that.
To open a case I recommend you to ask you lawyer. It is very complicated: some might take your content and cite it and it is ok. Some content might be so common, that you can't prove it is yours (for example: "An apple fell down the tree". How could I prove this is from me? Sometimes whole paragraphs are written like that).
After all I have always tried to use material on my website which is ok to be cited, copied, linked to whatever and it always worked. Even my best blog entries were not copied in a way I would had to deal with.
You could also try out a different path, if you don't like all the lawyers. Probably you want to publish content under Creative Commons:
http://creativecommons.org/licenses/ You could tell people they can use your content but they are not allowed to change anything and need to give you attribution. I mean, do you really want running behind people and look if they did not steal anything? I bet you would have better chances to allow them if they name you.
It all depends on the content i would say.
To summarize my answer: make a (c) notice and ask a lawer. Have in mind that different countries have different regulations, and using your content on a german server might be allowed even when it is not allowed on an US server.