Assess whether your work has been copyrighted. If your work has been published or you have worked to secure a copyright, it is likely protected by copyright law.
Contact an attorney and work with them to determine your legal options regarding the copyright infringement.
Write an email or letter to the editor of any publication in which you have seen your plagiarized work. Let them know that the material was stolen and they will likely remove it from their site.
Speak with an academic adviser at your university if you believe your schoolwork has been plagiarized. The adviser should be able to walk you through the necessary steps from bringing the issue to the attention of the university administration.
Keep records of everything you write, complete with dates. You should also store rejection/acceptance letters and other correspondence from publishers. This information can come in handy in the event of a plagiarism or copyright issue.