To become a judge first of all you will have to be a graduate in law.

The students from any stream can take up the law course. 

After completing your 10+2 studies you will have to give some entrance exams for doing the L.LB One of the most common entrance Being CLAT (Common Law Admission Test )

For appearing in the CLAT exam you must have completed 10+2 studies with a minimum of 50% marks. However a relaxation of 5% is given to the SC/ST/OBC and physically handicapped candidates.

The criteria for Most of the other entrances will also be the same. There may also be an upper age limit of 20 years for General candidate.
But these may vary from one entrance exam to another.

In CLAT exam questions will be asked from Legal aptitude, logical reasoning, English General Knowledge and elementary mathematics.

After completing graduation in Law you will have to give the Judicial service exams. These are state level exams conducted by the respective state government.

To be eligible for this exam you must have passed all the exams of L.LB in first attempt and also you must have secured a minimum of 55% marks in the final year examination.

The eligibility criteria may vary from one state exam to another.
The exam consist of Preliminary exam, Main exam and Viva Voice.

You will have to check the details of your respective state exams for further information.