West Bengal Civil Service exam is conducted in 2 phases (written):

1. Preliminary Exam – which serves as a Screening Test, consisting of 200 multiple-choice questions on General Studies.
2. Main Exam – When you qualify the preliminary exam, you appear for mains. It consists of six co mpulsory papers and one optional subject consisting of two papers (Only if you are applying for group A and / or B). There are two papers of the optional subject of 200 marks each.
3. Personality Test – Candidates, in order of merit on the results of the Main Examination for all the services and posts included in Groups A, B, C and D, then have to appear at a Personality Test, where he is asked questions on matters of general interest.

As you mentioned your choice of optional subject as computer science (Code - 19). So you must first go through the syllabus, for which you can consider the following:

Paper – I: Algorithms and Problem solving, Number systems and Arithmetic, Theory of Counting, Graphs and Algorithms, Boolean Algebra, Models of Computer Machines, Numerical Algorithms, Operations Research, Circuit and Network Theory, Basic Electronics, Instruments, Digital Logic and Systems, Data Communication, Data Structure.

Paper – II: Operating System, System Analysis and Design, Object oriented Programming, Computer Architecture and Organization, Micro Processor, Computer Networks, Database Management, Assembler, Loader and Linker, Compiler, Graphics and Multimedia.

Since you are in your first year of CSE, you will be introduced to these topics as part of your curriculum and make the most of it. Try to understand concept of these topics from the very beginning.

You can also consider this site : https://westbengal.pscnotes.com/ . You will find WBPSC free notes and current affairs here.

For reference, you can also check this link out: http://pscwbapplication.in/pdf18/ADVT-SCHEME-SYLLABUS-WB-CIVIL-SERVICE-EXE-EXAM19.pdf

Hope this information is useful. Good luck.