If you'd like to share your work with others, consider assigning a Creative Commons License to your work. This allows users to build upon your work easily and freely. And you won't need to explain and negotiate permissions. If you simply attach any of these license terms to your work, others will know instantly what types of uses you allow at no cost. It's all up to you!
BY = attribution required
BY NC = attribution required; noncommercial uses only
BY ND = attribution required; no derivative uses
BY SA = attribution required; "share alike" - meaning anyone who uses your work must share their resulting work with the same Commons license
BY NC SA = attribution required; no commercial uses; share alike
BY NC ND = attribution required; no commercial uses; no derivative uses
The most open option available is CC0.
CC0 = all rights relinquished to the extent possible by law
You've created a copyrightable work! Now, how do you protect it? US law protects works from their creation. You do not need to do anything other than create a protectable work to have copyright protection.
Nevertheless, registering your work with the U.S. Copyright Office offers advantages. Since Copyright Office records are searchable, registration creates a public record of your work's ownership and date of creation. This way, people interested in your work will know that you own the copyright. Second, registration is a prerequisite to a civil infringement action, should someone infringe your work. Also, you may be eligible for greater damages in the case of infringement if your work is registered.
You can register your work using this US Copyright Office online registration system. Note that online registration costs $35 for a single application and requires a deposit of a copy of your work.
Regardless of whether you register your work with the US Copyright Office, consider providing notice that you are the copyright owner. This is the recommended format for copyright notice:
"©" or "Copyright" or "Copyr." DATE OF FIRST PUBLICATION OWNER'S NAME
For example, © 2007 James Bond