My classes had prepared me for the job, but nothing had prepared me for office politics and the harsh, real-world dynamics of the professional environment. So, I take partial responsibility for the traumatic summer of my first real, full-time employment, simply because I was so green.
Still, I must admit that I learned a lot that summer, and now that I've worked more than seven years as a professional, I've learned a lot about what I should and shouldn't have done as an intern. Maybe you can learn from my mistakes.
Always have a pen and paper in hand. Be ready to take notes. Lots of notes. Write down everything, even if you don't think you'll forget. If nothing else, you'll appear very diligent.
Better to be quiet, and have people think you're an idiot, than to open your mouth and remove all doubt. I can't take credit for the phrase, but I've been saved by that adage on more than one occasion. And as an intern, you don't know what you don't know, so before speaking up in a meeting, make absolutely certain that you fully understand the topic of discussion.
Ask questions in private. One-on-one with your supervisor is best. If you're in a meeting and don't fully understand, it's best to take very good notes, and ask for clarification later.
Be on time. Better yet, be 10 minutes early for everything. It doesn't hurt if you work through lunch every now and then either.
Never make them ask you more than once. Strive to work ahead of people's expectations. Keep a to-do list, and don't forget to do anything that they've tasked you with. If your deadline is tomorrow, finish it today.
Make friends with the secretary. These nice ladies can have a lot of pull around the office. The same rule applies to the security guard, IT guys, facilities staff/janitors, etc. You never know when you'll need a favor, and these people will be just the ones to help you out. Start by smiling and saying "good morning." (These friends have helped me countless times.)
Double check your work. Before you turn in an assignment, double/triple checke for spelling/grammatical errors, misspelled names, miscalculations, typos, etc. Everything should look perfect because your version of "perfect" is your boss' version of "satisfactory." After you've worked a few years, you'll see what I mean.
Predict what questions your boss will ask. And be prepared to answer them. This is important when doing any kind of research on a topic. Know the answers to who, what, when, where, how & why. And if you don't know the answer to the question ...
Say "I'll find out." Instead of "I donno."
I wish I had known these things when I began my career. It would have save me a lot of grief.
As for my internship, I feel vindicated.
About five years after my internship, my resume somehow made to the desk of the president of the agency where I interned. She sent a recruiter to invite me to interview for the vice president position. I politely declined.
So, one day when you're the big wig, be nice to interns! They just might be your boss one day.