Here are a few thoughts from a soon-to-be senior:
Know your study habits (ie, studying alone vs group, flash cards, hand writing notes, etc) and stick to what works. Don't bother studying anything before getting into school, it'll be a waste of time. enjoy potentially your last summer off.
Try to get involved in groups that match your interests early, it's a good way to meet upperclassmen, who's advice can be really helpful. That said, it's not gospel, forge your own opinions as you go.
Always be friendly to everyone, learn people's names (classmates, faculty, staff), try to avoid talking behind people's backs.
Don't take a single bad grade too hard - try to assess what went wrong and avoid the same mistakes in the future. 4 years is a long time and will feel even more like an eternity if you stress over every single quiz and test. That said, prioritize grades if you feel there is any chance whatsoever you might be interested in specializing. Keep doors open.
Try to foster your hobbies outside of school if possible, school can be all-encompassing and it's easy to let hobbies or interests wane. I found that when I started taking more time off intentionally to go work out, play guitar, w/e, I studied more efficiently.
