I've probably sent off my last generation of high schoolers. I think I might miss it, or I might not. Here is the advice I always meant to tell all my kiddos.
You're in college to build connections.
Yes, learning and getting a degree and discovering yourself and finding a vaccine for malaria or whatever is important too, but you NEED these connections. College is the ONLY time in your life where you can actually apply to jobs and positions without letters of recommendation and previous experience- after that, you will need BOTH to get a job. If you don't get your letters and your experience and valuable work skills (i.e., lab techniques) NOW, when you can, what do you expect to do when you graduate?
You will not "have" time for what you want to do- you have to make time.
No one is joking when they say that time management is necessary for college. This also includes good study habits- don't amble along lightly and spend two hours studying what should take only half an hour. There are so many things you will want to do- ballroom dancing, study abroad, volunteering, having a significant other- and you should do them! But you are going to have to plan carefully and ahead of time.
College admissions is not an ending or a conclusion.
You're going to keep on working- you don't get anything just for getting in a college. You havenot achieved your dreams or your life goals just yet, and you are going to have to work hard for them. Don't think anything's set or made. Of course this is not saying that you need to spend your summer studying ahead just in case- please take some time off in the summer of your senior year! Go paragliding in Paraguay or take that ceramics course you've always wanted to do but never had time for! Keep working your bakery job so you'll have some extra money for food! Spend the entire summer doing nothing but chewing gum with your friends at the shopping mall! But this isn't a end, and it's just going to get harder.
You won't know what you want to do with your life. But you'll be expected to know anyways.
It's ridiculous but that's how things go. So think hard and do a lot of soul-searching, and PLAN YOUR COURSES AHEAD OF TIME.
It's okay to change.
Not a single one of my close friends went through college without changing their major or/and adding on another major or minor. One guy I know came in an environmental science major and came out a musician; another molecular biology major came out a dance major. I made a small change myself, from biology to biochemistry with Chinese Studies tacked on. Of course this takes planning, and you are going to have to think hard if you find yourself discontented. Majors aside, maybe some study skills that worked for you in high school might not work for you anymore. It's okay to change them.
HOWEVER, your changes need to be well-thought out and for good reason! You will not have free range to change willy-nilly. Each major requires its own coursework and classes and credits, and it will be very difficult, if not impossible, to suddenly declare an entirely new major for which you have no classes for in your senior year. You should NOT suddenly, in the end of your junior year, decide that maybe political science wasn't your thing and you want to tackle computer science.
You will be asked to do things you are not good at. That's not an excuse not to do well in them.
It's not like your GPA takes into account only the classes you're good at. Yes, you bio and chem majors, you engineers and you other STEM students, you are going to have to take that African American Literature class or that critical opera studies class. You might have to make a sculpture out of banana peels and be graded on it. Social Sciences and Arts majors, you are going to take that general chemistry class and "Well, I'm no good at chem" doesn't save anything on your transcript. People (ie your future hiring committee or grad school admissions people) are not going to look at a D on a transcript and see "Oh, this student's not good at very basic calculus." They're going to look at it and see "This student didn't put enough effort in. He or she is probably a slacker."
Every major and every path will have difficult parts.
No matter where you go, you will encounter hard parts on your road, but you should not turn back just because of one lumpy patch. "I didn't want to take Calculus" is not a valid excuse for not pursuing your dream of becoming a doctor.
Counselors, advisors, and other resources are there to help you do your work, not to do your work for you.
They are not an easy-way-out, but your college will probably have many valuable resources that can make things a great deal simpler and clearer for you. Of course, it's YOUR job to seek them out yourself and go to them andkeep going to them when you need to.
You already know you can do it, now do it well.
I'm tempted to say something cheesy here, but I'll spare you the pain.