hey im a high school junior, so ill be applying come the fall. itd be awesome if you guys could share any personal advice you have as well!
The two most important things on your college applications are your transcript and your essay. Presumably you've been working hard and have gotten as high grades as you can get, so I won't ruminate much on that.
The essay is probably the most important part after grades. This is really the only time they get to see your PERSONALITY and it's what distinguishes John Smith, 4.0 GPA, Captain of the Football team from Jane Doe, 4.0 GPA, Captain of the Soccer team. Don't write about what you think they want to hear, write about what parts of YOU is important to you. When I met my regional application reader, she knew a lot about me, including not only what I did in High School but who I was as a person. You want an essay that conveys your essence to your reader and makes it seem like they are meeting you yourself.
Most schools have a supplemental application that (other than BS like Princeton's "Your favorite word") includes a "Why _______?" essay. Don't be vague. At all. The worst thing you can do is talk in generalizations. If you're applying to Columbia, don't talk about a science school in the city. That's NYU too. That's Rockefeller too. My "Why Columbia?" essay talked about the Core--it's what makes Columbia distinct from other schools. Doing your research helps exponentially here. I talked about the specific classes in the Core, and if you can somehow work your research (maybe you're interested in a specific program or a set of classes, or certain opportunities) into your essay without it seeming contrived, it makes it even better.
As a final note on the essay: they know when you're BSing. Don't do it.
In terms of extracurriculars (which is hard to change as a junior but not _impossible_), you don't need to be "that guy who does everything". They're looking for depth, not breadth. Sure, I have a great friend from High School who did debate, fencing, multiple levels of orchestra, different academic teams and clubs, etc, and he got into Harvard. On the other hand, I mainly did fencing (showing lots of commitment going from JV -> Varsity -> Captain), an academic team or two, and a research project, and I'm typing this from my Columbia dorm. Show COMMITMENT. They don't care if you sampled every club or not. They want people who will contribute to better their university society, as well as bring diversity to the incoming class; showing commitment and passions for specific things, and following through on those passions, tells them that you have goals and will be driven at college. Maybe you'll even follow those passions through university.
Most importantly though, keep one thing in mind: there are more schools out there. Yeah, if I didn't get into Columbia I would've been upset. There's no real way to sugarcoat not getting accepted into your favorite school. However, just keep in mind that there are many schools out there that you will be equally, if not more, happy at. You will not only survive, but flourish at and, most importantly, ENJOY, the school you end up going to, no matter whether it is your first, second, or seventh choice. I can almost guarantee it.
Side note: any new Smogoners coming to Columbia? :3