Hello Ayushi,

Hope you're doing well.

For BA psychology,  the eligibility is 50% aggregate in class 12 with english as a core subject. So, you can pursue it irrespective of what stream you were in. Whether it be science,  commerce or humanities.

For the entrance exam, you need to have an idea of various fields of psychology. Basic english. Some of the college might even interview you. ( note: this is for masters in psychology,  since most of the bachelor's degree college take in admission on the basis of merit). A book called discovering psychology by zimbardo is highly prescribed by professionals.

And for BA psychology,  each university has a different curriculum, which is easily accessible through the official website of the university you're going to give the entrance for. But the basics include- verbal and non verbal reasoning,  english, social science subjects etc.

Hope my answer helps you.

All the best :)