Hii Shreshtha!
You haven't mentioned the name of the institution in which you aspire to pursue LLB after class 12th. But, let me tell you that , after class 12th you can opt for either a three year llb course or a five year integrated course. You need to opt for CLAT in order to get admissions in Law course providing institutions. It would be better if you restructure the question.and include the name of the institution in which you are interested to take admissions. The following link may help you -
https://www.google.com/amp/s/law.careers360.com/articles/9564-law-entrance-tests/amp
Hope this helps.
Good luck!