Just like prison, find the biggest toughest one on day one and smack them around to show who's boss.

Basically on your first day, there's always going to be that one trouble maker who's going to spend his or her whole time in school trying to push your buttons and influence the other students into misbehaving. The second the trouble starts, take the moment to stop the class(since it's the first day and no lessons are needed) and use child psychology lessons to get into the child's head by asking about their personal life. Find out what makes them tick. What their home life is like. Who his friends are. His dreams. His goals for the school year. His hobbies. What rules he lives by. And in the end, the rules you live by and what you expect of your students. That they should never be afraid to talk to you, to always feel safe in class, and that being a teacher is no different from being a student. You have to learn new stuff every year, there's always homework just 100x more, and you can't wait until summer vacation.