Well, I guess if the first was true, I would not be here teaching you. How do you become a teacher? Don't try to become a teacher. That is how it happened to me, I did not want it at all, but I was set up. Someone else wanted me to become a teacher.
Who should become a teacher? Well, actually that is a hard question. Certainly someone who is wanting to be "someone" should not become a teacher, because they would be motivated by the eight worldly conditions, especially Fame, Praise and Gain. They would probably not be a very good teacher because their intention would not be based in compassion for themselves and others. Sometimes, if one practices and tries to lessen the suffering in ourselves and is more motivated by compassion and wisdom, then Kamma decides this and we want to share our understanding with others.