Determine with certainty that you have not inadvertently plagiarized. Many students are misinformed about what plagiarism means. Most basically, it refers to representing someone else's research or words as your own. This is quite broad, and it's easy to accidentally commit plagiarism. Consult your school's statement on plagiarism for a precise definition.
Assemble all of your notes and supporting documentation to indicate that you conducted the research and wrote your submission without outside help.
Review your research and be ready to answer detailed questions regarding your work. Besides producing documentation, you must be able to demonstrate that you actually learned the material you presented in your paper.
Report to the meeting or the hearing with the board that adjudicates plagiarism charges. Dress well and behave respectfully, and generally represent yourself as a serious student who conscientiously completes coursework.