Self Study and Hard Work is the mantra. You need to study atleast 3-4 hours in a day and slowing increase it to 5-6 hours or more. If you are a CBSE board student , you NCERT textbooks are the best material to prepare from. Read each chapter thoroughly, do all the solved examples as well as all the exercises at the end of the chapter. Once you are fairly prepared start solving previous years papers.

If you are a state board student, same preparation as the above with your state board prescribed textbooks.

Remember hard work is the ony way out. Since you realise you wasted your time, I hope you start your schedule from today.

Best of luck