CAT

Please refer to following list for best MBA colleges in NCR: https://www.shiksha.com/mba/ranking/top-mba-colleges-delhi-ncr/2-0-0-10223-0. You can find all details for admission in these colleges.

In IIMs, you need to get above 98% and for the other college, the CAT score is above 85% to get admission.
All the best.

No, it is not necessary. You need to be maximum 20 years of age.
You can check the below mentioned link:
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi00c_w_YzPAhVEn5QKHVceC-UQFggkMAE&url=http://www.iimidr.ac.in/wp-content/uploads/Revised_AP_IPM2016-21_DomesticApplicants.pdf&usg=AFQjCNFOWWZiDlT0q8jhafcg2JwhplMogA

In case you have excellent verbal and numerical ability developed over the years in your basic schooling and graduation, you can prepare for CAT in 2 months easily.

yea it is compulsory

You can get admission. For this, you have to appear in CAT, MAT and XAT exam. You can give any one of these exams.

You need to submit your total graduation marks.

If you join now, you would at least need 4-5 months.

Nope, you cannot once decided.

Not at all. You just need to be honest and well-versed about what everything you've mentioned in the application form and other questions like "Why MBA?", "Why MBA from this college/university?", etc. The interview process is based on selection, and not rejection. So, unless you yourself screw up the interview, you are very likely to get selected. Of course, you need to have a great score(95+ percentile) to be considered for the interview rounds. Plus other factors also need to be in your favor, like good academic scores, work-ex, etc. Cracking an MBA interview is not difficult by any means. Good luck!