All these questions are highly dependent on what school you go to. Obviously Harvard is a lot more stressful than most other schools! I can speak for my experiences though - I attend the University of Southern California.
Well first, for the UK people - in America there isn't really a distinction between the words "college" and "university," they mean the same thing. Somebody said up there that college is typically 2 years, but I have never used it in that sense - that's a community college. We use the word "university" to mean the actual institution, while "college" is used in a more general sense, just meaning your years as a student in community college and/or university.
College is school with freedom. Nobody forces you to do anything, and your success purely relies on your own motivation and self-will - nobody gives a shit if you're not doing well in class, it's your fault. You sign up for mostly classes that you enjoy, and you basically form your four years. There are plenty of opportunities to get involved and pretty much no matter what, there are people who will share your same interests. I'd like to say that college lacks all the bullshit/drama/idiocism of high school, but if you take a step into a Fraternity or Sorority house you'll see that it's not true. The good news is you can choose to avoid these people if you'd like!
As for stress - yes and no. It's totally up to you how much stress you'll have. I know people who are in five clubs and sign up for everything - these people are stressed. If you balance your school and other activities you should be fine. You'll find that in college you have loads of free time between exam weeks
You can get a scholarship by applying to them - ask your high school counselor, that's their job! There are two types: merit based, and need based. So if you're smart and poor then hot damn you're in luck.
Yes you have to pay for a dorm, silly. You also have to pay for a meal plan if you live on campus. Dorm life is pretty friggin expensive considering the room they give you. It's a nice experience but if you don't want to blow your money on it, then it might not be a good idea. And you're usually not forced to have a roommate. There are single rooms available in university housing, and you can specify you prefer those. You can't always get what you want, though!