I was in your position this time last year, and I personally don't think that any preparation at this stage would be significant in helping you succeed at CAS.
Depending on your school's CAS policies, any amount of work done before the school year starts may not even be counted. And furthermore you can't start a CAS activity without getting approval from your CAS supervisor. So, if you start doing something right now and it happens to not be "CAS-worthy", then all of that hard work may go to waste.
I would certainly wait until the school year starts, until you are fully briefed of what to do, before you actually start doing it.
IMO CAS isn't particularly too taxing, and it's certainly one of the easier aspects of the IB in comparison to the EE and TOK, etc. I spent my first term of Year 12 (grade 11) sorting out my CAS, and once all of my activities are planned and approved, it's just a matter of doing them consistently throughout the entire 2 years.
You can certainly start thinking about what activities you want to do, but I wouldn't bother dwelling to much upon CAS right now. There are much more important and useful things to be doing, such as preparing for your more difficult subjects.