What I wish I'd known:
College is disorienting. It's very different. Some people (I was one of them) initially get really overwhelmed by exposure to the wider world: things like managing your own time, making judgments independent of your parents', encountering other people's cruelty, dealing with relationships, deciding among different professional/academic interests, staying hopeful and pragmatic despite tough competition. Growing up is tough and confusing and I think usually involves a lot of missteps and angst. I'm only just out of college and I certainly haven't got it all figured out.
But my advice is to take care of yourself -- do things that help you stay resilient despite all the confusion. First, get enough sleep, a nutritious diet, some exercise, and again ENOUGH SLEEP. Second, friendship is incredibly valuable, especially the friendship of stable, successful people you can look up to. The time you spend on friendship isn't wasted. Third, never miss deadlines and never skip class. Old-fashioned rules like that are a bulwark. You may be experimenting with new activities, or having personal drama, or going through a tough time: but if you never miss deadlines, you're quite a bit safer from real catastrophe. Fourth, nobody ever died from knowing too much math. Put some attention into learning things that you can use to support yourself in the future. This doesn't mean you should pick a "safe" major -- there are no safe majors -- but plan for the future, and acquire quantitative skills no matter what. Fifth, love yourself. Hokey, but very, very real.
Most people in the US with internet connections who are reading this site will at some point in their lives graduate high school. I haven't yet, and it seems like what I do afterwards will have a pretty big effect on the rest of my life.*
Given that, I think I should ask for some advice.
Generally,
Any advice? Anything you wish you knew? Disagreement with the premise? (If you disagree, please explain what to do anyway.)
More specific to the site,
Any advice for high schoolers with a rationalist and singularitarian bent? Who are probably looking at going to college?
Anything particularly effective for working against existential risk?
Any fields particularly useful for rationalists to know?
Any fields in which rationalists would be particularly helpful?
This is intended to be a pretty general reference for life advice for the young ones among us. With a college selection bent, probably. If you're in high school and have a specific situation that you want help with/advice for, please reply to this post with that. I think that a most people have specific skills/background they could leverage, so a one-size-fits all approach seems to be somewhat simplistic.
*I understand that I can always change plans later, but there are many many things that seem to require some level of commitment, like college.
Edit:
As Unnamed pointed out, also look at this article about undergraduate course selection.