There is a sharp difference between the preparation for boards and the preparation for JEE exams. The board exams generally test your capacity to remember while the JEE mains focuses on the application of the concepts you have learnt over the past two years. So, the best way out for preparation of both the exams is to go from the bottom up, i.e., "understand" the basics firsts followed by the higher concepts. This way you can answer well in both the boards and JEE Mains exams. All the Best!