Hi Naveen,

4 months are remaining for the first attempt of JEE Mains (In January 2019) and 6 months for XII Boards (March 2019). If you haven't completed your NCERT books yet, then I suggest you should be doing it as soon as possible. This will help you get good marks in Boards. For JEE Mains, you can appear again in April in case you didn't perform well in January exam. 

For JEE exam, prepare a time-table and start solving previous year question papers. Join a crash course from a reliable coaching institute if you feel you aren't prepared enough. This will at least help you know the important chapters of Physics, Chemistry, and Mathematics. So, pull up your socks and get started! All the best!

Hello,
You should first focus on clearing your basic concepts first from NCERT BOOKS.Then make a proper time table and give equal time to all subjects. Make sure you are consistent with your studies.Solve questions from top books like HC VERMA,RD SHARMA,OP TONDON and always clear your doubt from your teachers.Be motivated!!
Good luck.