Hello Priyanku,

     Admission into IITs for B.Tech is done through two stage entrance exams: 1. JEE Main 2.JEE Advanced.You have to qualify JEE Main first to be eligible for writing JEE Advanced. Selection of candidates into IITs will be purely dependent on the marks secured in JEE Advanced.

JEE Advanced is conducted by the seven Zonal Coordinating IITs under the guidance of the Joint Admission Board (JAB). The seven IITs are IIT Kharagpur, IIT Kanpur, IIT Madras, IIT Delhi, IIT Bombay, IIT Guwahati and IIT Roorkee.

JEE Advanced Eligibility Criteria:

JEE Advanced 2019 eligibility criteria are the list of requirements and conditions that the candidates have to satisfy to participate in the examination, Before filling the application form, the candidates are advised to check the eligibility criteria of JEE Advanced 2019. Ineligible candidates will be disqualified from the examination process. JEE Advanced eligibility criteria have been listed out below:

• Age Limit – General category candidates should have been born on or after October 1, 1994. Relaxation of 5 years is provided to the candidates belonging to SC/ST/PwD category.
• Qualifying Examination – Should have passed 10+2th standard qualifying (or equivalent) examination in 2018. Those who are appearing for the exam in 2019 will also be eligible for JEE Advanced. 
• JEE Main – The candidates should have qualified JEE Main examination. The cutoff for admissions into IITs should also be met by the candidates
• Number of Attempts – For JEE Advanced, the candidates will be able to attempt the examination two times in two consecutive years. 
• IIT Admissions – The candidates should have never taken admission in an IIT irrespective of whether they continued the programme or not or just accepted the seat by reporting at the centre. The candidates who have earlier joined any IIT but had subsequently cancelled their admission will also not be termed eligible for JEE Advanced.
  

JEE Advanced Exam Pattern:

• Examination Mode – Computer Based Test
• Number of Shits- 2 (9 am to 12 pm & 2 pm to 5 pm)
• Duration of Exam  - 3 hours
• Mode of Language – English and Hindi
• Paper in JEE Advanced 2019 – Paper I and Paper II. Each paper will be each divided into three parts – Physics, Chemistry and Mathematics.
• Sections – The three parts will each be divided into three sections – Section 1, Section 2 and Section 3.
• Nature of Questions – Multiple Choice Questions, Numerical Questions, Matching List Questions
• Marking Scheme – According to the particular question, the allotted full, partial, zero and negative marks will be given.
  

The article in the link will give you the opening and closing rank for computer science in various IITs.

https://engineering.careers360.com/articles/iits-cutoff-for-b-tech-computer-science-and-engineering

please do check and prepare accordingly.

All the best!!