Hello Shivam,

First of all you have to stay focused on your 12th board exam because you should have secure at least 75% marks in the 12th class board exam, or be in top 20 percentile in the 12th class exam conducted by the respective Boards. Then only you can eligible to appear in the jee main exam.

You can apply for jee main from the link given below

Https://jeemain.nic.in/webinfo/Public/Home.aspx

For preparation of jee main exam, the most important thing is your confidence after that we talk about syllabus. You should know the whole syllabus of jee main exam. Your basics in Physics, Chemistry & Mathematics should be very clear upto 12th std. You should read the NCERT books for basics. Then HC Verma for Physics, RD Sharma for Mathematics, OP Tendon for Physical & Inorganic Chemistry, and Organic Chemistry of "Arihant Publication" for detail or extra knowledge. 

You should also try to solve the previous years question papers, this helps too much and you should also give the mock test to rectify yourself and build your confidence level.

Thank You