Everybody at this school is smarter than you in some way or form. Don't be caught up in your own arrogance because you were top at your own school. Be humble, help others when you can, and they will help you when they can. MIT has a spirit of giving, and so don't be afraid to be seen as the dumb one - someone will gladly help you if you ask for it. Remember that you're not stupid, even when the p-sets seem unbearably difficult. You are at MIT for a reason, and you'll do well.
I knew it was going to be academically rigorous, that was a no-brainer. But I am still learning the importance about not letting academics take over your life. In freshman year, the course load was lighter and I was able to have a varied social life. However, first semester sophomore year I retreated to skipping weekend parties just to make more time to study. Again, I'm still figuring out the balance and I'm trying harder to prioritize friends over problemsets.
I wish I had known more about the community at MIT. In every class, the students get together to help each other learn. Outside of class, living and social groups offer a strong support system. Students come from all over the world and it's a bit intimidating to come to a new place, not knowing what to expect. The community helps alleviate these feelings. It may seem competitive to get in, but once you're in, everyone helps each other.
I wish I had learned about the resources available, like tutoring or reaching out to recitation leaders. I wish I had prepared myself better by learning more on my own. It was very difficult to catch up at the beginning.
Nothing. The journey is just as important as the destination!