Dear Vikas, there are 3 year Law Courses and 5 year Integrated Law Courses. There are certain Institutions which conduct Entrance Examinations while Few Institutions conduct admissions based on Merit. Either ways the highest qualification is 10+2. LLB is a 3 year Degree course that you can pursue right after your 12th. And then there are combination of Integrated courses like BA LLB, BBA LLB, BCom LLB and B.Sc LLB. For joining Law, there is an Entrance Examination called CLAT which is Common Law Admission Test. This is conducted by the National Law University. And then there are state level based Law entrance examinations and private institutional Law Entrance examinations as well.

Hello Vikas,
Yes, surely you can opt for law as soon as you complete your 12th. There is no restrictions for it. There are various colleges including govt as well as pvt.
For getting into top 19 national Law colleges you need to appear for CLAT (COMMON LAW ADMISSION TEST) held every year. Apart from this there are other various colleges across the India which are good some of them are Government Law College,Bombay, UPES Dehradun, Jindal Law college, Nirma law University, Ahmedabad and many more are there. Few of them hold there exams separately and few give admission on merit basis.
I hope this helps you.
All the best !!