Start your preparation from now itself. Read NCERT Textbooks of 11th and 12th standards of Physics, Mathematics and Chemistry. 

Focus on your Boards too in 12th standard. JEE Mains consists of Physics, Chemistry and Mathematics with 30 questions in each subject. A question if answered correctly rewards the student with 4 marks. If a student gives wrong answer then 1 mark is deducted. If question is not attempted then no marks are deducted or awarded. 

After completing NCERT textbooks you can go for reference books. Now there are many reference books so choose whichever you like and do it well. 

Practice Multiple Choice Questions of each chapter immediately after you finish  a particular chapter. 

Give equal importance to every subject as they are all equally important. 

Know the concepts well. Start from basics. When you know the concept then you can solve any question however difficult it is. 

If a student clears JEE Main then he/she is eligible to give JEE Advance. If a student clears JEE Advance with high rank then he/she can get admission in one of the IITs across India. 

By scoring high in JEE Mains student can get admission in engineering in NITs, IIITs, or GFTIs across India. 

If you still have any queries feel free to ask in the comment section down below.