Hi

No, you cannot get admission in NITs / IITs solely on the basis of marks obtained by you in12th board examination.

This is because,

12th qualification is just required to make you eligible for NITs/ IITs i. e one needs at least  75% marks in aggregate in class 12 if belong to general or obc category or at least  65% marks if belongs to SC/ST or  must be in the top 20 percentile in  class 12 examination but for this year i. e 2021, even this percentage barrier has been lifted off and now there's no minimum percentage requirement to be eligible.

Your admission is completely on the basis of rank obtained by you in JEE main examination in case of NIT and admission is on the rank obtained by you in JEE advanced examination in case of IIT .

So,

I suggest you  to prepare well for your JEE main and JEE advanced examination if you want to get admission in NIT and IITs following the below tips :-

*Make routine or timetable :-  your routine should revolve around things i.e study, revise and practice
*After finishing each chapter you should practice questions, you should practise at least 80 to 100 numericals / mcqs in Each chapter
*Solve previous year jee questions
*Once done with complete syllabus, go through jee mock test papers

To prepare well you can go for our jee mains knockout

https://learn.careers360.com/knockout-jee-main-april /

Thankyou