Do you have to go to a class so big it has to be held in an auditorium? This can be a bit difficult as the instructor would not be able to have enough time to answer everyone’s questions. Being in a large class can also be distracting.
Sit in front. This is always the best way to get all the information you need. Do this specially if you get easily distracted by all the people who will sit in front of you if you sit way in the back.
If you have questions that couldn’t be answered in class, you can always make an appointment to meet with your instructor during his consultation hours. Just make sure that you have everything prepared when you meet with your instructor so you don’t waste both of your times.
Try to get to know some of the people around you, even if the class is large. It is always nice to have people with whom you can form a study group. They can also help you when you get sick and not able to go to class.
Most importantly, attend class. It maybe a large one but some instructors value their students’ attendance. Some instructors also tend to give valuable insights on the subject matter rather that when you read them on your own.