To get good grades in class 12th is very important you should be dedicated and hard working.

In order to remember make sure your concepts are clear. Take help of NCERT books to build your basics strong.

To build your concentration you can prepare your own notes and flashcard. There should be no distraction while you are studying. You can take short breaks between your studies if you want.

Prepare a time table and follow that. Set your daily and weekly targets and try to acheive them. It will also help you to complete your syllabus on time and you will get enough time to revise your syllabus.

If you can't remember what you have learn then make sure your revise properly. Do revise the complete chapter next day it will help you to remember.

You can also solve MCQs related to the topics. So, that you can do enough practice which is very important.

Refer previous year papers as well for your practice.

All the best!!