As she sat in a high school geometry class and observed a beginning teacher, Matt (a pseudonym), teaching proof for the first time, the author was reminded of her own experiences in teaching formal proof to secondary school students. Matt seemed to struggle with some of the same challenges she encountered when she began teaching proof. For example, he seemed uncertain about which definitions and theorems would appear consistently throughout the course and thus should be emphasized. During her eight years as a secondary school mathematics teacher, the author found teaching formal proof in geometry one of the greatest challenges, so she decided to investigate the experience of learning to teach it. She did so by stepping into Matt's tenth-grade geometry classroom. To assist her in studying his practice, she videotaped lessons across the first three years (2005-2008) that Matt taught proof, and she conducted interviews to help her understand the choices he made in his teaching. In this article, the author reports what she learned through a combination of her observations in Matt's classroom, her own experiences as a teacher of geometry proof, and her reading of the literature on proof. More specifically, the author explores 10 things she wishes she had known about teaching proof in geometry before she taught it. (Contains 1 table and 2 figures.)