Build up the best possible relationship with the children. Apart from academic work, spend fun and quality time with them, exploring their interests and passions. You will be much more likely to talk effectively to them if the relationship between you is positive. Lead by example by communicating with them in a calm way. It is a lot to expect, but you are more likely to influence them if you are not just viewed as an angry teacher out to spoil their fun.
Listen to the children's viewpoints, but only if the children conform to polite norms of human interaction. They should speak in turn, should not shout and should not interrupt each other. Ask them how they, if they were the teacher, would deal with the way they have just behaved. They may surprise you with how strongly they would deal with misbehavior if they were in your shoes. Discuss the rules of the classroom with them.
Discuss consequences all the time with the children. If they fail to see the link between how they behave now and what happens in their lives, they will not see the need to change their behavior. Invite an appropriate person -- someone who was rebellious, got into trouble and later reformed -- to speak to the children. This can be an effective strategy.