What is the first thing a teacher should do when comes in to the classroom?

Greet your students cheerfully. I have a routine that I follow everyday. This gives order to your classroom and the students know what is expected of them when they enter the classroom.

The teacher should make sure that the room is ready and prepared for the first lesson.  There should be "Busy Work" on the students desk or table to keep them occupied while the teacher is taking attendance.

Introduce yourself in a way to develop an environment that is conducive to get your desired results as a remarkable teacher.

Greet your students, take attendance and talk about the lesson. Also Don't forget to Smile. 

Of course, greet the class, and talk of something lightheartedly to draw attention of all and create a positive atmosphere to generate interest. "I am going to teach this/that..." is too boring, you know! 

Say Hello , Class, I suppose ++++ :-)