My teacher thinks I plagiarized an essay; what should I tell him?

This is a tough situation. First and foremost, are you sure that you understand what plagiarism is? The short definition is this:
Any time you copy, borrow, or quote someone and don't give them credit, you are plagiarizing. Any time you turn in something that was written by someone else and pass it off as your own writing, you are plagiarizing.

Truth be told, it's pretty easy to accidentally use a paragraph or sentence from some other source and forget to footnote it.

Once you have a clear understanding of the topic, all you can really do is go to your teacher and point out that your intentions were honest. If you did accidentally forget to include a source, fess up. Don't try to build a case when you know you don't have one. Explain the oversight, or your mistake, and assure your teacher that it wasn't intentional. Be honest, and then all you can do is hope for mercy. Maybe your teacher will be a bit forgiving based on your maturity by confessing the mistake.

If, on the other hand, you are being falsely accused, don't be afraid to say so. Always be respectful, but assure the teacher that you — and only you — wrote the essay. Consider taking any and all research materials that you used to your teacher to prove your point. Again, be direct and honest; maybe more than any other time, this is not the time to brown-nose. This can be an intimidating prospect, but I'm sure that your teacher will appreciate your initiating the conversation.

If you're unclear about your teacher's expectations or what, exactly, his or her definition of plagiarism is, ask your teacher if the two of you can continue the conversation after school or during office hours. If nothing else, this will let your teacher know that you are serious about your studies and interested in the class — and that you want to go about getting your grades honestly.

Good luck.