When you first start in a new school then you need to very quickly take control of the class. This does not mean that you have to go in shouting and screaming at everyone on the first day, but you do need to lay down the ground rules.
Introduce yourself to the class and then go through some house rules. Explain what behaviour you expect from them and what will happen should they break the rules. Children respond well to rewards, so introducing a rewards system can help pupils stay on target. Traffic light systems have been used in many schools where pupils start off on a green light and then move to amber if they are on a warning and then to red if they do not improve which is then followed by a punishment such as missing a playtime.
If you find a group of children who are constantly misbehaving together then it is often a good idea to split them up.