Hii

For doing LLB one needs to posses a bachelors degree. So since you are in class 12th, You can go for some graduation degree agter 12th and then LLB. Or simply you can apply for 5 years integrated course BA LLB or BBA LLB as you like and save your 1 year. In this integrated course you get 2 degrees in a shorter time period.
For this you will have to appear in entrance exams like CLAT is for NLUs and other universities also accept it. AILET is for NLU delhi and there are many more university level exams.
https://law.careers360.com/exams

Thanks!!