Hi. I completed my Btech in 2016 and as of now working with an IT giant for past 8months. Shall I appear for CAT this year or next year ?

Since there is hardly any time left I suggest you to sincerely prepare this year and appear next November in 2018.
You should definitely join coaching Institute like TIME, CPLC, CATking or any of the mentioned names.
You can devote atleast an hour everyday on weekdays after work.
On weekends, I recommend atleast 6 hours of preparation and practice.
With full preparation and ample of mock exams, you will get full comand over CAT.
Meanwhile you can appear for exams like CMAT, CET, MAT in early Feb 2018 to gain some good hands kn experience of live Competitive exams.
All the Best.

As very less time is left for CAT 2017 and even the time for registration has also passed. So it will be good for you if you prepare for CAT 2018. Attempting next year CAT exam will not only provide you enough time to score very well and get a high percentile increasing your probability of getting a call from top b-schools of our country.

Along with this if you keep working along with your CAT 2018 preparations then you will be having around 2 years work experience. This will.help you in increasing your aggregate score which is used by every top college to make a merit list for calling students for next round of interview and WSAT.

Plus this work experience will also help you in your placements. In any IIM a student having 2 years work experience is ideal. Most of the companies looks for students who is neither fresher nor heavy work experience making 24 months work experience as an ideal figure. Also in IIMs final placements happen in two interval first is lateral placements in which students having work experience more than the average work experience of batch is allowed to sit in the companies which have a demand for students with work experience and then it is final placements.

IIM Bangalore and IIM Kozhikode are only two IIMs which give more weightage to work experience while selection of students. I am currently studying in IIM Kozhikode 2nd year and cut off for sitting in lateral placements last year was 22 months. This year placement is yet to start. So most probably you will get a chance to sit in lateral placements and if not able to get placed in that then you can again sit in final placement. So it's an additional chance and believe me all the companies coming in lateral placement offers way high above average package of your institute.

So in short prepare very hard All throughout the year with an aim of high percentile and get selected in on top b-school with your perks of work experience.

All the best!!