I want to prepare for UPSE then which subjects should I choose in ba

hello there!

You can choose any of the subject  as per your interest for Graduation, As per the eligibility to apply for the exam you shoould be Graduate in any of the field.

You can check the complete detail about the exam with the given below link.

https://competition.careers360.com/articles/upsc-civil-services-eligibility-criteria

Here are some preparation tips you can follow for the Exam preparation:
You will be tested through an objective paper in general studies. Be thorough with current affairs, if you are preparing from home then use ARIHANT magazine or any other famous current affairs monthly and memorise important points from it or attempt the MCQs given there. You must use pratiyogita darpan (english version) for descriptive answers or specific studies. Make notes in a notebook about whatever you read and then you will be on the right track. For language paper use class 10-12 books(level is matriculation ). Read THE HINDU or any other reputed newspaper and always go through the editorial column and make notes. For history civics and geography use NCERT of class 8-12. Be sure to do topic specific studies for important events like budget, census (questions are sure to be asked), any important socio-political event etc. Read political science books of NCERT properly. Study about economics, new events regarding technology, science etc.

NCERT books play a very important role for the exam so firstly prefer them.

i hope this helps you.

GOODLUCK!