BBA

Yes, having a work experience before opting for MBA is a good option. MBA is more valuable when you already have some work exposure. You can relate the concepts easily to the business.

I feel it is justified to analyse what you're doing right now. You must assess whether you are utilising every minute properly. For a young man, the present is very important, so begin the same, now. It is more important than what you do in the future.

Check these links-
https://www.shiksha.com/bba/colleges/bba-bms-bbm-bbs-colleges-in-west-bengal
https://www.shiksha.com/bba/colleges/bba-bms-bbm-bbs-colleges-in-jharkhand

MBA in IB is a very promising career option. If you pursue it from a reputed college, chances are there that you will get a good job. Salary is good. It is not necessary that you get a job abroad. You may have to sit in India and handle various international aspects of the business. There is a great scope of foreign tours.

It depends on your category. If you are a general/OBC category candidate then more than 95% is a good score. But marks are never a perfect barometer to judge in CBS. Sometimes a student with 80% score make it and sometimes a student with 98% can't make it. So, here the selection procedure not only focuses on marks but on overall personality of a person.
The application form comes out after the board exams. Keep an eye on newspaper after board exams.

BBA is general management studies and MBA is an enhancement of the knowledge. To pursue the career as a manager, BBA+MBA is the best option.
B.Tech is a technical field and students interested in the technical knowledge opt for B.Tech. Students after completing B.Tech work as the technical assistant and to move higher in the organisation they need managerial skills and hence opt for MBA. For B.Tech graduates, MBA opens more options as they have technical knowledge along with some managerial skills, hence benefiting them to understand the business in the better way.

Study as much as possible and score 80% and above. You can pursue your BBA and MBA from IIM-Indore. It conducts entrance examination for integrated MBA (BBA+MBA).
All the best.

You should appear for GMAT and then according to your score, you can apply for various colleges.