Class 12 is the good time to start your planning. Don't rely on marks .. Practice. To get 300+ practise .. practice.. practise..if you have already started preparing then undergo through revising. If you didn't start then it's ok. Start from now. Give 3 mocks daily on leave days and on school days give 1 mock a day. Revise the concepts that you have learned in the weekend. Practise previous year question paper once a week.. keep up good time table and go according to it. All the best buddy