Civil Services Examination is really big and requires long-term preparation involving many constituents from diverse disciplines.
So, right now, the best strategy is to keep reading NCERT books and strengthen your basics. Besides, cultivate newspaper reading as this will help you in gaining awareness and knowledge base and will keep you abreast with the latest.
It is too early to commence serious preparation for Civil Services Examination after 12th standard.
You can take civil Services Examination preparation seriously in the final year of your graduation and the solid foundation you are going to lay right now will be of great help as you advance.
Moreover, the nature of the examination is such that you need to be conversant with current affairs and contemporary developments so, whatever you prepare now will be of limited use or may become stale when you actually face this examination.
As such, maintaining tempo for such a long period reading, revising, updating is difficult and chances are by the time you become eligible, you may lose interest in it.