Hey
Admission to Four-year B.tech program is made through the Joint Entrance Exam (JEE) which is conducted jointly by the IITs, Indian Institute of Technology (BHU) and Indian School of Mines University, Dhanbad.
For taking an admission to BHU you will have to clear both JEE Mains and JEE Advance paper and only on the basis of your score and rank in JEE Advance you will be called for counseling.
Here is a link that will provide you more information about the exam:
http://www.careerlauncher.com/engineering/colleges/iit-bhu.html
All the best!