Find a group outside your department with whom you enjoy spending time. Recognize that you are surrounded by an elite peer group. Know that you would not have been admitted if you did not have the potential to become a competent student. Be aware that many people feel the way you do and are merely presenting a good front. Be kind to secretaries and shop workers.
Moderation is a very healthy concept.
Be humble and listen. Take some time to be alone. Carefully select a major professor, research topic, and committee. Meet with this person once a week to discuss your progress on your dissertations. Expect your graduate experience to be both pleasant and unpleasant.