Compared to classrooms in some countries, United States' classrooms tend to be informal.
There are, however, some very important basic rules:

Before class:

• Do your homework!
  Read critically; form your own opinions
• Review your notes
  from the previous lecture and reading for the day
• Communicate immediately with professors
  about any study problems
• Focus on the task at hand before class:
  take a moment of silence to gather your thoughts and mentally prepare yourself to the topic
• Write any objectives
  that come to mind at the head of your notepaper:
  • preparing for an up-coming test,
  • understanding a particular concept,
  • gaining a good foundation on a topic
  • understanding or reviewing the readings

In Class:

• Arrive on time for class.
  Professors do not take lateness lightly
• Position yourself in the classroom
  to focus on the subject matter;  consider the best location for:
  • listening
  • asking questions
  • seeing visual materials
  • discussing--not only with the teacher but also your classmates
• Avoid distractions
  that may interfere with your concentration
  (daydreaming, looking around the room, talking to a friend, passing notes, dozing)
• Evaluate as you listen:
  • Decide what is important and should be placed in your notes and what can be left out;
  • Listen long enough to be sure you understand what was said before writing.
  • Ask clarifying questions (but wait for "breaks" in the instructor's stream).
• Review your class objective(s) throughout the class period
  • Did your objective(s) mesh with the instructor's introductory remarks?
  • Has the class digressed from stated objectives, yours or the instructor's?
• Make a "to do" list including
  • assignments;
  • reviewing difficult concepts;
  • joining study groups;
  • making appointments with a study pal, tutor, or the instructor.
    One resource often overlooked is a classmate who seems to have a good grasp of the material. If it seem appropriate, seek the individual out for help.