Hi,
There are two type of programmes available 1. 5-year Integrated LLB programme 2. 3-year LLB programme. First option aspirant must have cleared class with at least 45% marks, Only those candidates can pursue 3-year LLB course who have completed Bachelor degree. As you are 12, you should go for 5-year Integrated LLB programme as it'll be more beneficial for you. CLAT is best entrance exam for taking admission in 5-year Integrated LLB programmes being offered by National Law Universities, these are considered best when it comes to 5-year LLB degree.

You can see List of Law Entrance exams in India and choose the best option for yourself. I hope it helps!