CAT

You are not eligible for appearing in CAT and SNAP examination if you are a general candidate.

Please refer this document to get an idea about the screening method and minimum requirement for IIMA (the best IIm college):
http://www.iima.ac.in/users/admission/files/Admission Procedure_2017_2019_Aug01_1.pdf
Similarly, each IIM will have it's own criteria, you can get this information on their website.

CAT is more difficult in comparison of MAT, but through CAT, you can get admission in a good B-school.

You need to score above 80 percent in graduation and 98 percentile in CAT exam to get an admission call from any of the IIMs.

Most of the exams are in English. Other than CAT, you can take MAT, IIFT, SNAP, XAT, CMAT, and NMAT exam in English.

You should join TIME coaching institute for the preparation of CAT exam. It has a good brand value, and its study material is also very good.

if you are completed your graduation or in the final year of graduation, then you are eligible to seat in CAT/MAT and need 50% marks( general category ) in past academic record. all the best

Not recommended, please go for full time and then pursue your certificate courses which shall aid you in placements.

First of all, start with the basic preparation like reading English newspaper. Solve mathematical problems daily. Try to score high marks in the first year because graduation percentage is also very important in CAT. The more you score the better your percentile in CAT. You can join T.I.M.E coaching institute in the third year or in the mid of the second year, but not before that.

You must first appear for CAT & MAT exams and after scoring your percentile, choose which college to apply.
As off now, you must focus on scoring good marks.
Best of luck!