you are eligible to pursue LLB course after the completion of your 12th standard/PUC. To join in LLB course, you should appear for CLAT(Common Law Admission Test ). If you can qualify CLAT exam, then you are eligible to get admission in LLB Undergraduate courses like BA LLB, B.Sc LLB, B Com LLB, BBA LLB, etc. These are five year integrated LAW courses, which you can do after the completion of your Pre-university.
If you are looking to pursue a degree in Law post your graduation then you need to enroll for a 3 year programme. Many universities offer this programme.
Hope this helps.