CBSE 12th

There are 20 functional IIMs right now in the country. Every year each of them releases their criteria for calling students for GD/WAT/PI rounds. Though academic performance at school does make an impact on a student's chances of getting a call from the IIMs, it is to be kept in mind that it's not the only criteria. One can always make up in other areas like CAT percentile, UG percentage or work experience. As far as weight-age is considered, different IIMs weigh the scores differently while calculating their composite scores for the selection process. Since you are still in the first year of your under graduation, try to score as high as you can in the same and later in the CAT.
Since the criteria are subject to change every year, it's difficult to answer what percentage will be required in the year you appear for CAT.

Hello Anjali,
I would suggest you to go for the graduation first. As you want a government job, the minimum eligibility criteria for the government job is graduation degree.
For the govt job after 12th, you can apply for the SSC CHSL examination.
There are many options for you can choose after the graduation.
You have great opportunity to make a career in banking sector. Apply for IBPS PO recruitment, IBPS Clerk, RRB officer, Specialist officer, etc. to make a career in banking.
After completing the 12th, you can choose following options:
B.Com
LLB
BA
CA
CS
Finance
Banking Sector Jobs: Apply for IBPS PO, IBPS Clerk, SBI, and many other private banks.
Insurance
Foreign Trade
After completing your graduation, you can be a CA, or CS, Postgraduate course (M.Com) or can apply for any private job profile in any financial institute, insurance sector, banks, etc.
There are many government exams like SSC CGL, IBPS bank exams, UPSC exams, civil services, etc., you can go for after the graduation.
Hope it will help you.