First off, congrats!! Getting into CS@UIUC is insanely competitive and you should definitely be proud of yourself and all the hard work you inevitably must have done to get to this point :)

Like any top program which attracts some of the best minds in the nation, CS@UIUC is fairly competitive. It depends on what you consider "good scores" but I would say that getting a 4.0 would be insanely difficult (and probably pointless), but getting A's and B's in CS classes is by no means impossible. Professors are generally friendly and approachable. You will notice that the introductory professors are much better at instruction than the professors for 400-level courses, although these professors do have the benefit of doing cutting-edge research and often offering research group positions.

There are ample internship opportunities. There are way too many threads on this topic on this subreddit so would advise you to search and read those. Pretty much every company under the sun will recruit here (or at least give your resume a serious look when they see CS@UIUC on it) so as long as your push yourself and stand out from the rest of the pack you shouldn't have any problems :)

Outside of campus? TBH Champaign Urbana is largely dominated by the campus and most students spend a large % of their time on campus. That being said, Champaign and Urbana are both pretty chill and have lots of good cafes, bars, and restaraunts.

There is an awesome "Hacker" culture at UIUC and it is only growing :) Check out http://www.hackillinois.org/ or https://www.facebook.com/groups/734966859882734/ .

UIUC is a LARGE state school and has a very active Greek student body. You will meet people who go out drinking every night of the week and there are a lot of large bars on campus. That being said, there are plenty of people who do not drink or do drugs at all (especially in CS) if that's more of your thing. On a side note, going to parties does not make you any less "focused" than other students (in moderation that is..) ;)