CBSE 12th

First you should decide what you want to do: BA or appear for the UPSC exams. If you are aiming to do BA economics you should try and score high marks in your 10+2 level examinations so that you are eligible to gain admission in BA economics in a college of your choice.
If you are interested in the civil services, you should devote yourself to studying for the UPSC exams,as this is no mean task. You will have to study at least 8-10 hours a day depending on your capacity to grasp different topics.
So basically, you should first sit down and decide which course or exam you want to give. You could also take the help of a career counselor to help you decide on a favorable course.

Dear Anjali Prasad, surely the science students can apply for the course. Integrated M.A. in Development Studies at IITM is an experiential learning based program. It has been designed in order to provide conceptual learning and knowledge based skills to students. Course pedagogy is designed uniquely in which students will be provided rigorous training. This 5 year integrated program helps designing a bright career in the field.
The students for the integrated masters program at IIT-M are selected through the Humanities and Social Sciences Entrance Examination (HSEE) conducted by IIT, Madras every year in the month of May. The exam is highly competitive and only about 2 percent of the candidates taking the exam are finally selected for the course.
To be eligible for admission, the student should have scored 60% (5% relaxation for SC/ST/PD) marks in aggregate in board examination and JEE(Main) & JEE (Advanced).
For further details kindly visit the below mentioned URL: http://hsee.iitm.ac.in/elig_crit.php.
I hope this would suffice. All the best and God bless you.