When you start a lesson, make your students understand clearly what they will learn today. You should have clear objectives, as such a plan will help students concentrate and know what they are supposed to do during this class.
You can write something like "do nows" on the board, or just tell them your plan step by step.
Source: https://busyteacher.org/16874-how-to-become-the-best-teacher-students-advice.html