This is what I remember about my first day of medical school... Let's just say I was a BIT overwhelmed. (The cadavers' faces were not covered, and THEY DIDN'T WARN US.)
Just know this: you'll be incredibly tired at the end of each day, but you will survive. And after the first round of exams, you'll actually feel confident that you can balance everything (enough to get by, at least). And by spring semester it'll feel normal, and you'll find yourself watching literally hundreds of episodes of Bones. (Or maybe that was just me.)
Have a great first day, new MS1's! And don't worry: it gets better all the time.