Hello Biswajit

First of all don't believe that you may not get a good rank in JEE Mains. Anything can be possible.
But considering your question - If you wish to enter the software line, do not lose your heart on not getting a good rank.
First of all you can give exams for various other institutes which have good CSE options such as BITS Pilani, VIT, Manipal Institute of Technology or SRM. 
Software engineering is quite a skill-based option. So wherever you go, do not stop learning because many aspirants take the software line making it a highly competitive one. Add more skills to your skill-set and perfect them too. Be aware of the latest developments in the techical world and stay determined. Specially in the software line, I have seen so many non-IITians or non-(some big colleges) guy making it big in the software line.
Hope this helps !
All the best !

Hi,

First of all be positive and try hard t secure good marks in JEE Mains. If unfortunately you do not succeed in achieving good marks and if you want only C.S than there are many good universities which conduct their own exams like

1.BITS PILANI

2.THAPAR UNIVERSITY

3.VIT VELLORE

4.SRM

5. MANIPAL

AND many more.....