You’re correct that you can’t choose your dorm as a freshman. The only options you have for that are to select that you want to live in a residence hall (in which case they will randomly assign you to either CKB, Mews, High Rise, Low Rise, Donlon, or Dickson), a townhouse (sort of like apartment-style living while still being on campus and on the meal plan), Balch (the all-female dorm, definitely the best dorm on campus in my extremely biased opinion - it’s where I live), or you can apply to live in a program house (each is different and is centered around a theme such as music, theater, ecology, etc.)
So there’s no way to ensure you will get into the residence hall of your choice, which is the problem with gambling and selecting residence hall as your top pick, because some are definitely better than others. As for a single, if you put down that as your top choice, I believe you’re fairly likely to get it.
As far as picking a roommate goes, obviously if you’re planning on living in a single you don’t really have to worry about this. But there will be a Facebook page for your class dedicated to finding a roommate (ours was simple called "Cornell Class of 2018 - Find a Roommate). There are also different websites dedicated to helping students match up with and talk to potential roommates.
There should be a checklist coming out online that lists all of the steps you have to take to ensure you’re ready for move-in day. I’m not sure exactly when that comes out, but it’s very handy and checks off all the steps you’ve completed and tells you the timeframe for everything you have left to do.
When it comes to stuff to bring, I would suggest just paying attention to everything you use on a daily basis now. For the most part, the things you need now and the things you will need in college won’t change too much, as far as basic clothing/toiletry items go. If you’re from a warm climate, make sure to invest in winter clothing. Buy a really warm coat and boots/gloves/hats in the summer when they’re on sale. I’m from Ithaca, so I already had everything I needed in terms of clothing, but a lot of people were freaking out because they didn’t know how to dress warmly. Other things you may wish to bring would include a laptop, school supplies (pens, pencils, notebooks, etc.), a bedspread, a shower caddy, shower shoes, decorations for your room, a mini-fridge, and probably some other things that I can’t think of right now. There will be a list Cornell posts of suggested and prohibited things for your dorm.
If you have any other questions at all, or would like me to elaborate further on anything, please let me know! I would be more than happy to.