Copyrighting Your Own Work
A work is protected by copyright as soon as it is "fixed in a tangible medium," (assuming it is an original expression) but even so you may wish to officially register your work with the U.S. Copyright Office, as registration is necessary before you can bring a suit of copyright infringement, and timely registration gives you the right to claim statutory damages in an infringement suit.
You may also wish to consider licensing your work instead through the Creative Commons. A Commons license allows you to decide which rights you will keep and under what conditions you will allow others to use your work freely.