Class 11 and 12 covers all the basic concepts that are required in the college studies. That's why, Class 11 and 12 topics are asked in various competitions for getting good colleges. 

First of all, you should have one very good source where you can cover all the theoretical concpets very nicely. It may be in school or you can watch our lectures at learnohub as we are providing free coaching for NEET and JEE.https://www.youtube.com/@LearnoHubClass1112

Second, you need good books to prepare and solve questions. A few of the recommendations for the books : https://www.youtube.com/watch?v=fT-WqbRpd5w

Third, you need to solve previous year questions of your subjects related to the competition you are preparing for.

Fourth, you should attempt mock tests regulary to check your progress.
Feel free to reach out to us for any problems related to studies.