Hey Pushpa!

If you're referring to the B.A. programme psychology, then you may opt for complementary subjects like sociology, philosophy, english, political science,etc. Usually these subjects go in good correlation with that of psychology. Although it's upto your interest as in which subject you are interested in and want to know more or study more about it. Also,if you have goals set for your future,as in what course you want to go for in further studies then you may choose that subject which will be an important aspect in your goal. Just choosing any subject is not enough. You must ensure that you would want to study it, then only you will be able to perform better in it. If in any case, you are referring to the honours course do mention it in the comment section.

All the best!