Answers

It all depends on what you want to study. Most schools have a specialty, or at least an area where they're superior or have better professors. Institutes of technology like CalTech or MIT focus on Math/Science (technology), whereas liberal arts schools will focus on humanities (English, history, philosophy, etc.) I'm applying to lots of liberal arts schools because I want to keep away from Math and Science forever. ihate hate hate them, but thats just me. find your own likes and a school to match!

certain schools have a reputation for having distinct social lives, and you can find out about those things on the school's website or just form your friends?

I'm applying to really social schools like Miami and UF. In general, state schools have bigger social scenes because there are more students. private schools can have social scenes, too, but theyre smaller so theyre not as socially diverse usually.

In general, state schools are just different versions of each other. They all offer pretty much everything and have a lot going on on campus. Private schools differ a lot in what kind of classes/majors they offer and how quality their programs are. If you want to go to a private school, its best to look at the websites/literature from each one. otherwise, you can't be sure what youre getting into

there are lots of places that rank schools or categorize them based on what they offer/how good they are. US News and world report is a magazine that puts out a list of schools every year with rankings, and I think princeton review does it too. US News is generally better and more quoted in articles and stuff, though, according to my college counselor.