Dear Satyam,

There will be no weightage for the 12 class marks in calculating the ranks in the JEE (main) examination.However,for the candidates to qualify for the admission in the IITs/NITs/IIITs and such other CFTIs whose admissions are based on the JEE(Advanced)/JEE(Main) ranks,they should have secured at least 75% marks in the  12th class examination or be in the top 20 percentile in the 12th class examination conducted by the respective Boards. For SC/ST students the qualifying marks would be 65% in the 12th class examination.

For further clarification you can visit JEE Main-2018 official site, shared below:

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

All the best...!!!!