At the beginning of the lesson split the class into teams, for a simple example team A, B, C, D (if they are sitting at tables maybe each table is one team)
Begin the lesson and give the clear instruction that you will count down from 3 and that they should be quiet at the end of the countdown.
You can demonstrate this to make sure it is understood.
If they are sat quietly at the end of the countdown add points to their team score, if they are noisy or any other undesirable behavior take away points from their team score.
At the end of the lesson, make a big deal about congratulating the winning team, you can give a prize if you are feeling generous but usually, just a high five or something is enough.
Eventually, they will get used to the rules of the classroom and understand the benefits of good behaviour.