IIM

Hi Adrita,
IIM Bangalore as such does not have any minimum cutoff percentage in class 10th, 12th or Bachelors, which you need to clear. However, it assigns a weight of 50% (20% for 10th, 10% for 12th and 20% for bachelors score) and thus, it would be difficult to get a call from IIM Bangalore (refer to http://www.iimb.ac.in/sites/default/files/iimb-pgp-admission-process.pdf). IIM Ahmedabad on the other hand has a minimum cutoff percentage which is set at 80 percentile score. Thus, looking at your graduation score, it would be impossible to get a call from IIMA (unless you are in top 20 percentile at your college, please refer to https://web.iima.ac.in/users/admission/files/Shortlisting Criteria for PGP 2017-19 batch_110117.pdf).
However, having said that, there are many top B-schools who will consider you provided you could score a respectable score in CAT. Therefore, your aim to be to maximize your CAT score. All the best.

For getting into IIM, first you need to attend for PI/What rounds conducted by IIM's. For getting a call from reputed IIM's you must possess a good percentile in CAT exam. Good percentile could be at-least more than 90 for general category.
These play a major role in getting a call from IIM. An average scholar with at least an aggregate of 85% in X, XII and 65% in UG can get a chance to get into IIM.
One should be able to speak fluently in English, which helps to defeat the others in GD (in some cases) and it also helps in PI as well. Because one needs to understand what the interviewer is asking and able to communicate his/her views clearly. Next comes to writing, where in you need to write an essay on the topic given. For mastering in that round, you should know the correct grammar usage and good comprehension.
Moreover, for a successful conversation of an IIM call, it is depends on your abilities too like convincing skills, answering skills, etc. Also, there may be many factors such as work experience, academic achievements, etc., which do not count for minimum requirement for getting into IIM but if you do possess them, they tend to an added advantage.

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.