I wish that before I started mechanical engineering I knew that having a degree in the subject doesn't mean you'll actually be any good at it in real life. The degree only focuses on the maths/analytical side; there is a whole world of engineering beyond just using equations. I wish that before I started engineering I had gotten some short work experience at a more hands on or practical level. This would've greatly helped me in both my internships.
Regarding internships, what do you want to know?