That is, comparing your posture to the next person is meaningless. Eyes may be open or shut. Regardless, the focus should be inwards. Avoid eating or drinking for two hours before class. An empty stomach will allow you to breathe properly and free the body's energy for the task at hand.
Breathing is integral to yoga.
You will be instructed when to breathe in and out. Your in breath should be nasal and the out breath through the mouth, strong and audible. You will be asked to breath out during that part of the exercise which requires effort. The in breath corresponds to the relaxation phase. Your breath should be deep, controlled and abdominal.