How do your lessons start? When I work with individual teachers, we always look at what I call the First Five Minutes.
It is the beginning of the lesson proper — the moment when you need just a little bit of attention from the whole class to explain what they will be doing for the next fifty minutes or so. This slice of time is critical. I have seen many lessons where a teacher did not get these first minutes quite right, and paid for it for the rest of the session.
It does not need to be five minutes. Just two minutes with the whole class listening — and I mean the whole class, not most of them, not all except one — is worth more than twenty minutes of instruction to half a class. When you only secure the attention of some students for some of the time, you spend the rest of the lesson re-explaining the activity over and over. Do this regularly and your class learns not to bother listening, because they know you will come to them in a moment anyway.
Before the formal start there is the settling-in time — the moment just after students stream through the door, energised or unsettled from whatever happened before. The best approach here is to wait at the door, say hello, and direct students to a simple activity already waiting on their desks or on the board. The activity must be easy to access for everyone and require no explanation. If it needs explaining, do that at the end of the previous lesson.
Sometimes I just write a question on the board — something with no right or wrong answer that anyone can attempt immediately:
Start as you mean to go on.