CAT

Yes, you can, but YouTube isn't exactly the best place to learn about management. Here are a few channels:
Takshzila Shikshak
BYJU's
Career Launcher
Hand Ka Funda.

Yes, but you need to take Amity Written Test on the day of the interview.

Here are a few suggestions:
IIM-K
IIT-C
University of Calcutta
NIT, Durgapur
Goenka College of Commerce and Business Education. SIT.

The word 'good' varies from people to people. But I'll give you a general answer. If you're thinking about old Iims (A, B, C and L), you have to score about 98 percentile with a good background. For the colleges just below old Iims (SP Jain, MDI, etc. ), you have to score above 95 percentile. Background matters as well. For the new Iims, above 90 percentile is necessary.

To be very honest, all are more or less the same. What a coaching institute will do is, it will make you go through the exam syllabus and patterns. At the end of the day, it's your effort that matters.

Hi, You can start preparing for CAT 2018 as well as some other exams like CMAT NMAT, XAT etc. If you are applying for this year, this is the high time to join a coaching institute or study yourself.

Hi, The gap year doesn't matter if you crack CAT but first, make sure that you want to go for MBA. Get a clear idea about the specialisation. Take CAT if you are really interested.

See, if you are interested in both. Then, it will cause a problem for you. I'll prefer you to go and discuss each and every aspect with your elders and the person who is in this field, which your are confused in. As far as I have seen opting for the GATE is better for you as there is job security for you.

From which college? Top colleges require 99+% for admissions in various colleges.