At the beginning of the class period or school day there are a lot of things to accomplish. Implementing a starter activity, taking roll, greeting students at the door, locating or creating a seating chart, setting expectations, etc. One thing that should be a priority is setting a positive reinforcement strategy or a system for rewarding appropriate student behavior.
By focusing on the positive rewards you are increasing the likelihood of students behaving well. Some rewards include a fun game for the last few minutes of class, handing out tickets during class and using them for a drawing, or something else.
What reward system do you have in place for when you substitute teach?