In be eligible for NITs/IIITS anf GFTIs you should have 75% marks in your board exam or you should be in top 20 percentile.

For SC/ST you should have 65% marks.

There is no weightage of class 12th in JEE Main exam.

If you want to score good marks in JEE Main make sure your basics are clear. Make sure you read NCERT books throughly. So, that you can solve tricky questions easily. 

You should be aware of the complete syllabus, exam pattern and important topics.You should complete your whole syllabus and focus on important topics more. You can solve as many as MCQs. For exam pattern and to know level of questions you can refer previous year papers.

Solve complete mock test series to work on your time management and to increase your speed. With the help of mock test series you can analyze your preparation level and you will get to know your strength and weakness. So, you can work upon them.

Revision plays very important role in scoring good marks. You should revise your syllabus properly. You can make your important notes as well which will help you in revision.

All the best for your preparation!!