Absolutely tell him face to face. That will make him respect you more. Here's what I would say if I were in your shoes, "Dad, I have something to tell you. I've been smoking Marijuana for the last <x> days/weeks/months/years. I know that it is nothing to be ashamed of because you do it, but I felt that I should tell you about it, because I respect you enough to not keep things from you. I hope that you will show me the respect that I have given you, and continue to allow me to smoke this herb. As I have shown of late, I can still smoke marijuana and be a successful teenager in school and such."

If you say that, he'll probably let you smoke still, and you'll have a better, stronger relationship with your father. Let us know how it goes.