Hello aspirant,
I am here for helping you......
As in your doubt it's not clearly mentioned that  which university you are searching for so, as per the general record I am sharing you a link below where you can have complete detail of admission process on the basis of your class 12th marks .
https://engineering.careers360.com/articles/btech-admissions-on-basis-of-class-12th
And there are so many entrance exams  separately for all universities by which you can take admission in these universities .
Now for taking admission , a common exam is JEE mains which is acceptable by many universities/ colleges and for many state universities they also conduct their separate exams
For example - UPSEE for having admission in UP state university (AKTU or UPTU).
If you want to know the admission process for any particular college/university so, you can mention it in the comment section below.
I hope it will be beneficial for you....
For more queries you can upvote my answer and can ask in the comment section.....
Best of luck......

Hi dear
You didn't mention the name of the college for which you want to know the admission procedure.
For most of the colleges, the admission is on the basis of the jee mains rank, you have to apply online and attend counseling and can get admission on the basis of your rank.
Some colleges do conduct entrance exams for the admission .
Some private colleges even provide direct admission on +2 basis.

Please do mention the name of the college for other details.

Best of Luck!!