How can I prepare for CS in university
I'm not well aware of study pattern in your region, but I can advise you according to my own experience of 6 years in this field. I am in post graduation course.
One of the many things you need to know is CS is not all about programming, although most of the concepts you can relate to it and easily understand. Most importantly, its useless to learn every language, but mastery in any one language would be very helpful. Slowly you'll start understanding that all you need to understand is declaring variables, loops, functions and some built-in or library functions for a language, and it'll do most of your basic work.

Secondly, theory alone is of little use, except in the field of algorithms. From programming to digital and networking, you need to understand the practical examples. You need to start from "How will I do this?" then go for "what are the problems in my ways?" and then you'll almost always end up with adopting standard ways of doing things, this methodology will give you a lot of help.

Nothing is too difficult if you work hard and follow right approach. Computer science is a field where either you'll understand things or you won't. There is little to no use of understanding half things. You can't know half algo. You can't know OSI architecture upto 3 layers. You'll forget things if you do them that way. But once you have struggled enough, things would start relating so nicely that you'll easily be able to forecast solutions to a problem in one subject due to your experiences from other subjects.

My advice would be to go for the course if there is some curiosity in you when you think how facebook works? How Windows work? How am I connected? Why is everything 0 or 1? WHAT is 0 and 1?
All the best.