IB Senior here!
CAS hours are Creativity (C) Action (A) and Service (S) hours that make a candidate well rounded. You need 150 total: aim for 50 of each letter to be balanced.
Creativity are things like art clubs (marching band, national art honor society, newspaper); Action are pretty much sports or exercise; Service is volunteer work. There is overlap between the CAS, and you get to chose what you want your hours to go toward.
You end up writing CAS Reflections in a Journal (your school will go over this in great detail!) and explain your goals, successes, failures, and general outcomes. There are a series of CAS "Outcomes" you will have to include and reflect on in the hournals, but you will get a list of these. You want to show that you infuse the IB Characteristics in your life. (The characteristics are inquiry, caring, principled, reflective, communicator, ETC).
The best way to prepare is to enroll in a couple extra curriculars you can really dedicate yourself to. I did a ton of music stuff, which is mainly creativity. In order to get some action hours, I used some marching band performances in the action category because I know I am not sporty. It's good to have things over lap like that in case you need flexibility.
Another requirement is the Extended Essay (EE). It is a 4,000 word academic essay (a lot shorter than it sounds). You will have a supervisor and start it later in your junior year, probably. You can write it about pretty much anything you want, but the requirements/expectations vary a lot between the different EE Topics (Sciences/Lab Work versus English). I wouldn't worry about this right away.
Theory of Knowledge (ToK) is a required class for the diploma. You take 2 semesters. It is basically a practical philosophy. You have 2 IB assignments in this course, but you will focus on them in the class for the most part. The class aims to make IB students well rounded, open minded, analytical thinkers. "how do we know what we know and why do we know it?" is the gist of the class.
In each IB Class, you will have internal and external assessments. Internals are done throughout Junior and Senior year and will be guided by your teachers. Externals are the exams you take in may of your senior year.
It's a lot to take in at first, and it will be hard work for the Diploma Program, but I think it will be worth it in the end. Colleges are practically lining up at my door! Of course, it isn't that way for every student, though. If you have any questions or concerns or want me to elaborate on anything, feel free to post here or PM me! Good Luck this year!