NIT

The validity of the MAT score is 1 year so you can apply with the score next year.

The best option would be to join a coaching institute. They will tutor you accordingly and they will helo you to understand complex concepts while preparing. Be prepared to give your best and study your BCA course thoroughly too. Study at least 2 to 3 hours daily and solve practice papers.

It will be from 2019 onwards.

The announcement by MHRD, the revised criteria of 75% marks (Gen Category), 65% (SC/ST/PwD Category) or being in the top 20 percentile in class XII is likely to be applicable for admissions to all NITs/IIITs/GFTIs along with IITs from the admission year 2017-18 onwards.

YouTube is a good source; you can get it from there, so go through it.

All Indian nationals who secured at least 60 per cent marks in aggregate of 6.5 CGPA or more on a 10-point scale in their graduation*, are eligible to apply for NIMCET 2017. For SC and ST candidates, the minimum eligibility will be scaled down to 55 per cent marks or 6.0 CGPA on a 10 point scale! If you meet this criteria you are eligible for NIMCET. Good Luck!

It's not a compulsion that a student can get placed with IIT or NIT. It depends on the skills of the student and ability to expose his/her ideas and also on technical knowledge.

You can refer to the following books, these books have basic as well as advanced concept and easy to understand. Excel With Objective Book For NIMCET by J B Dixit, Ruchi Sharma, Ashish Mangal
Upkars MCA Test by A Team of Experts
Arihant MCA Entrance Test (Self Study Guide) by Admit M. Agarwal
Mathematics General Aptitude Books by R S Agarwal. Barron's Military Flight Aptitude Tests by Terry L. Duran
The Aptitude Test Workbook by Jim Barrett
Numerical Ability & Mathematical Aptitude by Dr. A. B. Rao
Good luck.

I think you should do CS IT from iiit Gwalior. But it also depends upon your convenience and interest. You can get insight of both and then take decision. Best of luck.