Hey.

After you've completed your Class 12th and want to become an IAS officer, you need to complete any Undergraduation course first to be eligible to take the IAS exam. You can attempt the exam in the final year of your degree hence you cant start it's preparation during you degree too. There are three stages in an IAS exam- Preliminary Examination, Mains Examination and Personal Interview. The Prelims exam will include General Knowledge and Aptitude. You need to clear the prelims in order to qualify for the Mains exam. You need to choose one of the subject of your choice for the Mains paper. Hence you can choose any subject which you have an interest in.  If you choose to pursue your UG degree in any specific field, you can choose that too. Candidates who clear the Mains exam sit for the Interview. And the final selection is done on the basis of merit in mains exam as well as the Interview.

Best of luck