The first thing that you need to do for subject selection is to look at all the groups and subjects in the syllabus. Second things that you want to do is to make a list of subjects that you like. This could be based on your interests. Then go and have a look at their past papers. Once you have a list of subjects, then I can guide you more.
Just make a list, don't bother about combination or anything. For now you just want to know what subjects appeal you.
Also, remember that CSS is a leadership exam, and the final decision must be your own.