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You don't "get out of it." If you did it on purpose, say you are sorry, and that it won't happen again. If it was an accident, say you will be more careful in the future.
Prove you did not plagiarize. Show your sources and how you cited them. Prove your thoughts are original....otherwise you are screwed. Most colleges and Universities have zero tolerance for plagiarism.
If your professor uses a service such as "Turn it in" then you could plead ignorance (incorrect citation). You could also show drafts of your work. NOTE! Most plagiarism checkers are now banking everything. So if you ripped from your old paper in highschool you HAVE plagiarized yourself. In which case produce the original paper to show that you did that. You may not get full credit and you may be failed but atleast you won't be expelled.