In most cases, I am a pretty organized person. I have the calendar on my phone set up to plan out everything I'm doing for the foreseeable future. I have all my classes and meetings, and parties handled so I know what is when and exactly how much time I have in between these events. I like having my workplace neat, so I clean it regularly. Organization is important to me, but that does not by any means suggest that somebody who doesn't organize can't prepare.
But first, lets assume you are somebody who likes staying organized and planning everything in advance. Preparing your schedule is an obvious thing, as well as buying everything you'll need beforehand. Phone calendars usually work pretty well for scheduling purposes (plus you won't have to carry anything new or actually write anything down). If you're like this, then you probably know how you like to handle things. Just make sure that you don't procrastinate and you handle all of those things sooner than later! I typically don't buy school supplies until after the first few days (beyond basic stuff like pen and paper). That way I know exactly what I'll need. That may not work for you. If you don't like the idea of rushing to get your materials then you should probably go buy stuff now using your best judgment.
On the other hand, if you're not generally very organized, that's perfectly fine. Neatness is only a necessity if you need it for peace of mind, but if you can operate without it, then do it. (Mostly in life you should just probably do what works best for you.) You don't want to go to class without knowing what you're doing, though. You still want to bring pen and paper, even if you don't have a binder! You can buy stuff that makes your life easier later. You don't want to be lost, though. You'll at least need to have a game plan for how you're getting to and from school (assuming you can't drive for whatever reason). Things like that. Of course you should have an idea of what needs to happen and what you need to do to accomplish it, regardless of how 'neatly' that list fits anywhere. Basically, if you don't plan ahead, you just need to know yourself and how you deal with things. Be prepared for whatever may come.
I guess the best way to do get ready for something is to know how you handle situations. If you're an organized person, don't forget to prepare and put things in neat boxes. If you just go with the flow, make sure you give yourself a flow to go with. Don't let things get in the way of establishing your comfort zone. So, if you need to make something happen before school starts, don't sweat it and handle it now.