Hey Shivani.

There are two kinds of Law courses-

3 year LLb where you have already done your graduation separately. You pursue Ba and then later on go for 3 year LLB. Will take you total 6 years.

5 year Law- Where graduation degree like BA is integrated with a Law degree and both are completed together in 5 years. You can take admission in this course directly after Class 12th.

As far as chosing subejcts of BA in either of the course is concerned, you must do that as per your own interest. The choice of subjects in BA will not affect your performance in the Law course in any way. Hence it is always better to choose any subject that you want. Various colleges give the option of subjects like History, Political Science, English, Economics, Sociology etc. Your subjects in Ba wont affect your performance in the law courses and hence go with the one you have an interest in. 

Best of luck