I use to be a university hired tutor for a few of the first year programming courses for Math and CS majors. I was responsible for marking assignments and exams, as well as running labs and being available after lectures to answer questions. Near the end of my employment, I was also a substitute lecturer for when a professor couldn't make it for whatever reason. I did this for nearly 3 years. So my advice is from personal experience.
If you are starting off as a marker and TA, then you can get a feel for the course. You'll get to see where the students are struggling. Get a copy of the course notes and read them before you mark anything. Take some time to do the assignments as well. It will give you a better understanding of what they are doing and how to better help students understand the concepts.
If you can, attend the lectures. Again, knowing how and what they are being taught will help you understand how it is being presented, and also allow you to come up with your own teaching style of the material.
Before you are given a class, hold office hours. Teaching students in small numbers when they have direct questions is easier than teaching a room of students with varying degrees of understanding and motivation. This will also help you practice presenting the material in a way that students will understand.
Doing it this way will provide you with a solid foundation of the course material and give you your teaching tools for properly teaching it. Take note of the questions students are asking in office hours or at the end of lectures. If there are a lot of questions on something, be sure to revisit it next time and to change how you present it in the future.
It's a learning experience for you as much as the students.
I know, I didn't really cover much of the what to teach/how to teach it concepts that you're looking for. But, if you follow these tips, I feel that you'll be able to teach a CS course no matter the topic (provided you have experience with said topic).