When you first walk into a classroom, take note of what is going on and interactions between the teacher, students, and other staff.  Put your belongings to the side and ensure you have your hands free.  Briefly look to see if the teacher has a place where paras check-in or could gather information for the day.

Check the schedule - know the subject being taught and student's learning objective.  These things can usually be located on/around the front board.  Identify what kind of work the students will be doing (whole class, small group, independent) and any upcoming transitions (to recess, music, end of class period, etc.).

Most importantly, be mobile around the classroom.  Increase proximity to students who are off task, redirect undesired behaviors, encourage students to participate in instruction, and check-in on student needs.  You might offer reteaching to students or assist the teacher in passing out or organizing materials. 

Seek an opportunity to check-in with the teacher.  If the teacher is not able to step aside, seek information from other classroom staff.  Ask how you might be able to support them and the students, if they need anything prepared, or if there are behavior plans to be aware of.  This should NOT interrupt instruction.