The course that was perhaps the greatest revelation to me was my PHYS400 course. It is an accelerated course that combines the standard modern physics course and the quantum mechanics course, taught by (in my humble opinion) one of the department's best professors. We've covered a huge amount of content, apparently more than both of the courses it's replacing combined. It's been the most self-study oriented course I've yet experienced (and honestly may end up being the most self-study oriented of all the undergrad courses I take.) This has allowed me to develop my mathematical and physics abilities, as well as my teaching ability (teaching yourself can be as hard as teaching others.) More importantly, it's helped me develop persistence in solving hard problems that require creative thinking and development of theoretical frameworks, which is exactly the type of work I hope to do in the future. I think my favorite topic so far has been the coverage of spin, because spin 1/2 systems are simple enough to concisely but clearly represent the bizarre properties of quantum mechanics. I discovered PHYS400 through my friends in my fall physics theory course (PHYS273.) To that end, I'll give the obvious answer of a strategy for new students: make as many friends in your major as possible, from as broad a sampling range as you can in that major. This will not only help you locate opportunities like this one, but increase the depth of your conceptual understanding and the size of your support network if you're having problems in your major.
My two greatest successes in developing relationships with faculty have been with my professor from PHYS400, and with my professor from PHYS265 (a Python programming course for physics majors.) I've been lucky enough to have plenty of questions for my PHYS400 professor after class, and he's been very patient in answering them. My advice that I can pull from that relationship is that I've learned not to be afraid to engage in discussions of interesting ideas with my professor as if they were my (albeit MUCH more experienced and knowledgeable) peer. Academics are where they are at least partially because they love nerding out about their field, and if you can reach that space with a professor consistently then forming a relationship with them will be easy. My PHYS265 professor was also my PHYS273 professor last semester, and I've been able to keep up that relationship both through contact after class and through my TA position for PHYS273 this spring. He's been very helpful in overcoming common issues one has teaching that class, and with general advice about being a teacher. To state the obvious, if an incoming student intends to work under or adjacent to a professor, they should develop a close relationship with them as soon as possible. It's not a bad idea to test the waters with all your professors, so as to find the ones who seem willing to help, and have a track record of working with undergrads.
My relationships with other students have of course been crucial to my adjustment to college. I loosely break the friends I've made into two functions in my social support network: I have friends who have helped me adjust to being a physics student, all of whom are in my major and many of whom are older, and I have friends who have more helped to substitute for having a family or dependable friends. The former type are the more obviously useful ones, but it goes without saying my favorite part about being friends with them is having friends fascinated by the same things as I am. You'd be hard pressed to find one person in my high school fascinated by physics the way I am, and here they're almost a dime a dozen. You'll have moments where leaving home doesn't feel like an adjustment at all; it's like you've finally found your crowd. The other type of friends play a quieter but perhaps more powerful role: they're your firm support network. I think it's really important to have friends with whom your relationship doesn't depend on continuously sharing interests, but on a basis of simple shared experience and trust. These friends are there for you, even when you irritate each other, which has obvious utility; more subtly, you're there for them in the same way, which is a great exercise in the other side of adulthood, namely living partially to support other people. I hope in answering this question I haven't come across as extractive; ultimately, making friends is something you do unconsciously and is much more emotional than the rational way I'm depicting it here, but I do think there is some rhyme and reason to how and when we pick our friends. I think incoming freshmen should be most of all on the lookout for people who make them feel safe, not cool or smart. Good friends are a constantly grounding reality check that remind you that you live in a real, interpersonal world. I'd say the biggest obstacles I've had in forming these relationships have been time and inertia. The former is obvious: you're so busy in college that things like extracurriculars often fall by the wayside, and with them your perceived bandwidth for making friends. There have absolutely been people who I enjoyed myself around that I simply got too busy to keep following up with, and though it sounds terrible, forgot about. Inertia is also real: college is such a big life change that it can feel grounding and safe to shrink your social network down to a consistent 5 or 6 people, which is part natural and part just the lazy part of your brain. It can be a dangerous habit to become content with who you already know.
I was least prepared for the long term. I managed to escape my first semester without learning this lesson, but I've learned it this semester: there comes a point where it's no longer fair to yourself or even efficient to keep pushing yourself. I'm speaking both about time (leaving time for sleep and occasional entertainment) and about your emotions and self-concept. The two tie strongly into each other. I went into last semester with a mindset that I had very lofty goals and would need to be exceptionally hardworking to achieve them, which is somewhat true, but led me to the feeling that a second not spent explicitly getting ahead of my classmates was "wasted." This is most clearly wrong in the sense that a life where you can't afford to think about anything but your work is no life at all, but more subtly wrong in that it's simply incorrect in how success is achieved. Success is achieved by the accumulation of your effort and interest over long stretches of time, and the latter can't be maintained if you make what you love a constant life-or-death fight. I've learned to take myself a little less seriously, stress a little less, and let myself play, both with physics and with the rest of my life. More concretely, I've also learned to sleep. I've learned about myself that when I consistently get less than 6.5 hours of sleep, my total productivity is less than if I had just gone to bed; you pay the cost the next day, when you're too tired to make yourself do anything. In summary, chill out, sleep enough, enjoy it while you're here, and don't try to maximize (that includes trying to maximize fun. Try to give yourself time that's neither highly productive nor as exciting as a party.)