Talk to the class instructor if possible. If you are a student with knowledge of a fellow student's plagiarism, the most appropriate course of action is to speak to that instructor directly.
Speak with the student directly if you are the instructor. Before you file a complaint, see what the plagiarizer has to say for himself. He may beg you not to report him or he may accept what he's done. In some cases, the person may not even be aware of the plagiarism and will cooperate with you after speaking to him.
Compare the text to the plagiarized one. Outline the similarities and exactly what points were stolen. This will help you make the case when you file a complaint.
File the appropriate paperwork through your school's judicial affairs or discipline office. Most larger schools will have an office set up specifically to deal with legal problems at the school. They will ask you to fill out paperwork and submit the plagiarized essay.
Consult a lawyer if the plagiarism offense happens outside of a school. A lawyer can help guide you through the process of suing the plagiarizer. You will have to prove that they profited off your creations, which is considerably easier to do if you hold a copyright for your work. If you insist on representing yourself, the necessary paperwork can be picked up from your local small claims court and filed for a small fee.