Which exam is to qualify for btech after 12th
JEE Main is the National level Entarnce Exam to pursue the engineering programme, There are other state level & other university level exam also, You can apply. In order to get admission in IITs, NITs, IIITs, GFTIs you should have minimum 75% in class 10+2 as a general category & 65% as a reserved category Or you should be in top 20 percentile in your class 12th exam conducted by the respective board. You can check the further eligibility with the given below link.
https://engineering.careers360.com/articles/jee-main-eligibility-criteria
Good Luck!