I did CS HL and got a 7. Currently enrolled in a MSci Computer Science student at University.
Computer Science HL comes down to your programming skills. Programming is not something you can learn in a couple of weeks. It is a subject that grows on you. Start with the programming straight away, and do it a lot! Having a mini-project every week is gonna help you through it. eg. hangman, sudoku and more advanced stuff like chess or checkers when you get the hang of it.
The theory bit is ok, but the discrete mathematics can be challenging for some.
Also your Paper 3 is a case study. Beware that you will need to spend a lot of time doing research and being all on your own, since you teacher wont necessarily know the topic any better than you initially do. Remember that there are no relevant previous papers for this.
The IA is also a bit different than other Group-4 subjects, whereas in the other sciences you just do a big experiment - in CS you have to design, develop and implement a system for a real customer. It is fairly straight forward as you are mostly graded on the documentation and implementation, and not the programming itself.