Hello,
Even if you appear for JEE mains, you won't be eligible for admission into NITs IIITs and GFTIs. Also you won't be eligible to appear for JEE Advanced.
If you belong to General category you need minimum of 75% aggregate in your class 12 boards (65% minimum for ST/SC/OBC). Apart from that, if you are in the top 20 percentile of your respective boards, then also you are eligible.
So, if you qualify in JEE mains, you can take admission in only private engineering colleges.

You can appear for the 12 boards improvement exam to increase your class 12 marks.
You can either for all subjects improvement exam or a particular subject in which your score is low.

For more details please visit this link
https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp&ved=2ahUKEwjTybSxkNbqAhUjwzgGHQ1rAI0QFjADegQIAxAB&usg=AOvVaw0Ddf__yVIUe0laa-JjEit-&ampcf=1

Hope it helps

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