IIM Lucknow - Indian Institute of Management

Your academic performance up to class 12th is excellent and hope for a much better performance for B.Com also. The last year's IIM-Lucknow cut-off for CAT is given below for an idea and for future preparation for the General candidate.
For the PGP program the sectional cut-off was 85 percentile for VARC, DILR, and QA respectively, while overall was 90 percentile.
Further, the class 12th marks, graduation marks, work experience, etc. are important factors contributing 40% weightage in the selection of a candidate. The CAT percentile plays a vital role with 60% of weightage in the selection of a candidate.
So, you have to start your preparation accordingly.

Going for MBA can have two directions, one you can pursue MBA as a fresher and work or two you gain some work experience and later, figure out what to do. If you want to go the first way, I believe you should appear for CAT 19/20, and check what is your current status compared to other aspirants around the country. It will give you a rough idea of where you stand. You have around 5 months which is good enough for you to prepare. If it doesn't work, you can get some experience a year or two and then try for CAT once simultaneously. So basically its a question you need to ask yourself. But whatever it is, I would recommend you to definitely appear for CAT 19.