Hey there

A simple graduation is all you need to sit for UPSC civil services if you are talking about civil services no matter from which stream. You can take any subject in your graduation but if you want to pursue BA I'll recommend history, geography will be best as in interview you can't be cross questioned as why you want to change your profession after graduation also you will be completing some syllabus of upsc during BA. But it's completely your choice what you want to choose.

Good luck

If you look at the entrance eXam model for UPSC, it is a 3 stage   selection process involving Prelims, Mains and Interview which tests your personality test. Prelims has 2 Papers namely General Studies and CSAT.

General Studies has Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and Social Development, Environmental Ecology, Biodiversity and Climate Change and General Science.

CSAT has Comprehension, Interpersonal skills including communication skills, Logical Reasoning and Analytical ability, Decision making and problem solving, General Mental Ability, Basic numeracy and Data Interpretation.

So to clear the first and 2nd hurdles, you can take up subjects like Political Science, Geography or History in your Graduation. That way you will be well prepared for the topics asked in Prelims and Mains. But remember that you will need atleast to start preparing ahead of 1- 2 years. If you are preparing at home, you need to keep yourself away from distractions and focus on the goal ahead. Gather as much as information as you can on each topic and keep yourself updated. Practise mock test papers.