First of all you must read the eligibility criteria and see if you are fit to take the exam.
Preparing for CSS is a grueling process as it requires putting in a lot of study hours and utmost dedication.
Once you figure out that you can take the test, list down your subject strengths/ or subjects your are already familiar with.
Since there is a lot of material to cover, it is always wise to pick subjects that you are either good at or that you have studied at some point in your undergraduate years.

Hope this helps a little bit