iam in third year b.tech student cs stream.i wants to do IAS exam preparation with art side subjects.how i start the preparation?

Select subjects do a detail study and practice previous years papers

I will advice to opt for Engineering subjects for IAS. Being science & engineering stu subjectent, you may not find Arts subject intresting. Anyway, to start with arts subject you can select History, Political science, Geography or anyother subject of your interest or possible interest.