12th standard is a good time to start preparing for Law examinations. Rather than targeting NLSIU or CLAT, you should focus on learning all concepts tested for law examination selection. Adopt a well-structured learn-practice-test approach.
Focus on clearing your concepts first. Once you are through, solve different Q types and take CLAT mocks. Analyze the mocks thoroughly. This will help you improve your score.
One of my friends graduated from NLSIU and these are some things he suggested that might help you with your preparation.
Since you have almost a year of preparation ahead, use the time to your benefit and follow a systematic learn-practice-prep approach.
First, learn all concepts tested across law exam sections. Once done with concepts, solve as many different quantitative aptitude questions.
Finally, solve time-bound mock tests for CLAT as per the new paper pattern and analyze every test before moving on to the next one, for continuous score improvement.