Hi Aparna,
     There are many options available after completing 12th std. Since, you are an IAS aspirant , I would suggest the following :

Humanities-Humanities have traditionally been termed as the bastion of civil services examination.

Engineering-If we take IAS toppers from last few years, it is clearly evident that candidates with engineering background have dominated the examination.

Science- A small but respectable number of candidates from graduates from science stream do take up civil services and crack it regularly. However, their number has not really been great enough to leave an impact on the final tally.

B.Com / MBA / BBA / CA / CS / CFA

Even if you pick the ‘so-called’ ideal steam or the subject, it will not matter until and unless you really are passionate about it. All the best!!!!