Review your test carefully. Look over the questions, your responses and the instructor's comments, to make sure you followed the guidelines.
Read the information about testing and grades in the course syllabus. This information may help you understand your grade. For instance, if the syllabus says plagiarism results in an automatic fail for the assignment and you plagiarized on the exam, you probably have little basis for an appeal.
Meet with the instructor as soon as possible. Take your test and syllabus with you to refer to during the conversation. Explain your concerns clearly and logically without resorting to threats or emotion.
Follow the procedure outlined in your student handbook or college catalog, if you don't get satisfaction from talking to the instructor -- or can't arrange a meeting. These publications explain the process for appealing grades. You will probably have to fill out a form to explain the situation for a formal appeal.
Attend any required meetings during the process. Skipping a meeting indicates that you aren't very serious about the issue and hurts your chances for success.